Until a few weeks ago, the pupils of Cotton Spinners Association Secondary School had never heard of Hugh Bonneville. Downton Abbey may be a worldwide television phenomenon, viewed in 220 territories, but word of its success hadn't reached them in Kwai Chung.

When it was announced that Bonneville was going to pay a visit, the students prepared themselves by watching Paddington. In the film, Bonneville plays Mr Brown, a family man who picks up a Peruvian bear at London's Paddington Station; as roles go, it's not entirely typical of his oeuvre but, as the school's founders might say, it's an excellent yarn.

Bonneville was in Hong Kong to promote the British Academy of Film and Television Arts (Bafta), which is keen to strengthen its links with Asia and, particularly, China. As part of his trip, he went to a couple of schools. Cotton Spinners, a band-three school, was chosen because of its connection with Shakespeare4All, a charity that helps pupils perform the Bard's plays to improve their fluency in English and whose founding sponsor is Swire Properties. (No relation to Lavinia Swire, whom Downton fans will remember as the young woman to whom Matthew Crawley was engaged until she died of Spanish flu, leaving Crawley free to marry Lady Mary, father an heir, meet with a fatal motor accident and ruin Christmas Day 2012 for millions of appalled viewers.)

"Hugh will go into the group and read some lines from the play and try to teach them how to act," explained the school's principal, Lau Kit-sim, just before his arrival. Behind her, about 30 children could be seen interacting on stage, under a banner stating "Learning English Drama with Hugh Bonneville". This turned out to be a rehearsal for the photo shoot that would take place at the end of the hour-long visit. Soon, the call went up for everyone to grab their scripts.

The play in question was Macbeth. On hearing that Bonneville mightn't like to hear its title spoken aloud - actors being famously superstitious on this point - Luk Mei-yan, who's in charge of the English drama club and had just pronounced the dread word, looked a little stricken and said, "We won't say I said it. So you should call it 'the Scottish Play'? I like that!" The advice was rendered useless shortly afterwards when Bonneville, consulted on the point, yelled, "I think we can say MACBETH!" (Later in the afternoon, asked about such rashness, he explains, with the air of a man making it up as he goes along, "It depends on which country you're in.")

Contrary to real-life celebrity shrinkage, at 1.88 metres, he's taller than you'd expect; the extreme grandeur of Highclere Castle, which stands in for Downton Abbey, has a miniaturising effect on humans. In Cotton Spinners' assembly hall, he was the loftiest person in the room. He'd just come from a band-one school on Hong Kong Island, talking to about 200 11-year-old girls; apparently he'd been slightly nervous about these visits but was getting to grips with the new role. Two decades ago, as his career was getting under way, he'd briefly played a headmaster in British soap opera EastEnders and perhaps he was channelling both that experience and his more recent, upscale, soap-opera turn as Robert Crawley, Earl of Grantham and dispenser of patrician goodwill.

At any rate, he'd decided the students should explore the concept of hierarchy. "One really important thing in this play is status," he told them. "Who holds power, who's envious of power, who'll go to extremes, like murder, to achieve power … In Shakespeare you can't just be a king, it's how you create the air of kingship."

A giant pack of playing cards was spread over the floor and each pupil selected one, glanced at it and began behaving according to where in the deck it came. This was a quick lesson in the school of life's arbitrariness: choose a jack or a queen and you're up in the drawing room with Lady Mary; pick an ace or a deuce and you're down in the kitchen with Mrs Patmore. There was some obsequious bowing and extravagant strutting, plenty of hands-across-the-mouth giggling, occasional whispering in Cantonese.

"Guys! Guys! Guys!" cried Bonneville mildly, as everyone (guys and girls) shuffled into place according to station. "We aren't using words, it's about behaviour. It's about non-verbal communication - eye contact, demeanour."

The lesson moved on - deftly aided by tutor Sanjay Ramburuth, from Shakespeare4All - to Macbeth and the scene in which Banquo's ghost appears. Ramburuth appointed a First and Second Murderer to stand by, along with a grinning Ghost. Bonneville left to make his regal entrance.

"You are now in the banqueting hall of Macbeth's castle," announced Fiona Kaaka, Shakespeare4All's managing director, to the students seated in two long rows as if at a medieval table. "And who will it be coming in? Will it be Mr Hugh Bonneville? Will it be Mr Paddington Bear? No! It is the king, Macbeth!"

By the third re-enactment, Bonneville's shrieking and groaning - as the Ghost (now swayed by the seriousness of the occasion) pointed accusingly at him - had reached impressive levels. To the astonishment of the children, he fell to the floor and lay, whimpering, in a foetal position.

"The guy who came in was king of the world," he explained afterwards. "And now he's turned into a gibbering wreck."

A short, respectful Q&A ensued. Someone asked him about the difficulties of his job.

"Recently I was playing Lord Mountbatten in a film," he said. "He's a very significant figure in history and I found it difficult to understand him as a man, his political activities and attitudes. And then I met his daughter and she talked about playing with him as a child and there was a connection. It may not be real but it becomes true."

One of the boys wanted to know how he could become a big star.

"Anyone can be a star nowadays, it seems to me, if you turn up on reality TV," responded Bonneville. "I'm an actor. Ultimately it's down to what's inside you, and if you can hear the word 'no'. Rejection is a huge part of being an actor." And he went on to talk about his early days at the Royal Shakespeare Company (RSC). "I was a spear-bearer, I wasn't even a fairy. I was the understudy for Lysander [in A Midsummer Night's Dream], who was played by Ralph Fiennes - he's now M in James Bond. Then I got promoted to Lysander when we went on tour and he became Oberon."

What he didn't tell the students was that after 2½ years, his RSC contract wasn't renewed, an episode in his life which, he has previously said, left him "devastated". Acting, to him, meant theatre not television and film, and after the RSC dropped him, he thought he would never work again.

For a while, in the early 1990s, he did bits and pieces of television work. In those days, he called himself Richard Bonneville. His real name is Hugh Richard Bonneville Williams; he wanted to keep Hugh Williams as a writing byline but friends were getting confused about whether to call him Hugh or Richard so he had to decide which he was. After that 1995 EastEnders headmaster role, he stuck with Hugh Bonneville; from then on, he started to make a name for himself.

IN LIFE'S GAME OF CARDS, Bonneville, 52, was handed a high number: surgeon father, private school (Sherborne, in Dorset) and Cambridge University, where he read theology and once played Lord Macbeth in an undergraduate production.

"Awful, dystopian, student bollocks," he says, in one of the Peninsula Hotel's meeting rooms. "It certainly wouldn't have won a Bafta."

He and his wife, Lulu, arrived from West Sussex the previous afternoon. Since then he's been the star-bait at a Bafta fund-raising dinner and visited the two schools. Later, he'll do a Bafta "In Conversation With" event at Elements.

The Guardian newspaper once described him as being "as solid and plummy as a warm Christmas pudding"; just now, however, he's at that mid-afternoon, jet-lagged point when what he most closely resembles is a Hong Kong taxi driver at 4pm, simultaneously showing For Hire and Out of Service signs, and wary that you'll want to travel somewhere he'd rather not go. Already this interview has been cut to 30 minutes and the booked photographer has been dismissed, unused. (For the visual record: he's in an open-necked, pale-blue shirt and cream linen jacket, looking a tiny bit long-suffering in a noblesse oblige sort of way.)

Also in the room, along with a representative from a local PR company, is Amanda Berry, Bafta's chief executive and the person generally credited with having revived its image. In 2001, Berry - now 54, and pleasantly enthusiastic - shifted the British Academy's film-awards ceremony to a time slot ahead of the Oscars, whereupon it became a glossy harbinger rather than a dusty also-ran. These days, she's focusing on Bafta's global image in Asia. China, obviously, is vital; Bafta's Hong Kong Advisory Board was set up in 2013 as a mainland hopping-off point, and during Xi Jinping's recent visit to London, Prince William - Bafta's president - presented the Chinese president with a painting to encourage Sino-British creative collaboration.

Bonneville is part of this stellar push. In 2013, Eddie Redmayne (who went on to win a Bafta for The Theory of Everything this year) came out here to give a masterclass at the Hong Kong Academy for Performing Arts.

"We've been asking Hugh from the beginning but he's always been busy," Berry explains, loyally. "We couldn't have a better ambassador - he's four times Bafta-nominated! We knew he'd be incredibly comfortable with everything we threw at him."

This year, Bafta filmed its own Downton Abbey tribute, which will be shown on British television over the Christmas season. Officially the series has ended but on Christmas Day there will also be a Downton Abbey special, a kind of epilogue to all six seasons. Will Bonneville be sitting down to watch it?

"We've got rather a lot of people staying this year," he says, in a manner that wouldn't disgrace Lord Grantham himself. Not wishing to inflict the show on guests, he thinks he might just have it on "in one of the rooms".

Over the years, the Downton household has had its issues, beginning with the 1912 sinking of the Titanic. (The telegram announcing the watery demise of two Crawley heirs in episode one was given to Bonneville as a parting gift by the set designer.) Along the road, Robert Crawley has occasionally behaved in ways that have prompted Bonneville to email Julian Fellowes, Downton Abbey's creator and writer, querying his lordship's IQ levels.

"He was pretty dim at times and I was quite surprised by that," he says.

The correspondence I'd be having, however, is with the person who designed the opening credits: every episode kicks off with an extreme close-up of a dog's bottom under the words Hugh Bonneville. What did he think when he saw that for the first time?

"Well, I did ring Gareth Neame, the producer, and ask if it was locked in." ("Really?" asks Berry, with a laugh. Yes, he's being serious.)

Possibly his most memorable moment of the 52 episodes occurred last season, when his character's gastric ulcer burst spectacularly during a dinner attended by future prime minister Neville Chamberlain. The British media compared it to the movie Alien; as it happens, I interviewed John Hurt (the original stomach-exploder) years ago, who said the first intimation his fellow cast members had that something unusual might be brewing was when they arrived on set to find the camera crew in mackintoshes. Were the macs out at Downton?

"Oh no, everybody knew, we'd rehearsed it for three weeks beforehand," says Bonneville. "And there was a doctor talking through the colour of the blood."

In case you're wondering, this wasn't because it spouted blue; the initial projectile-gush, having pooled in his lordship's stomach, had to appear darker red than fresher gouts in case medically savvy viewers complained.

"I didn't give a stuff about the colour," he says. "But I had to get the spatters right because it would take 45 minutes to reset the table properly."

Correct table settings are another ongoing obsession with fans of the show. There's also been Asparagus-gate (his lordship cutting the vegetable with a knife instead of using fingers), Double-Yellow-Line-gate (not painted on British roads until 1960) and, indeed, an actual Water-gate (plastic bottle c. 2014 left on mantelpiece in publicity still for 1924).

The series, being about class, has tapped into a reliable strain of British loopiness. When I ask whether Bonneville has ever been to Grantham, he shakes his head, wearily. Former prime minister Margaret Thatcher was, famously, born a shopkeeper's daughter in the Lincolnshire market town, and as Fellowes adored Thatcher, I've occasionally wondered if the earldom was a private joke …

But to this theory, Bonneville sighs and remarks, "We're always getting letters saying, 'I've got a presentation mug in the attic that says from Robert Crawley to my daughter Sybil'. We always say, 'They're not real people.'"

Mountbatten, of course, was. Bonneville's resemblance to that particular lord is negligible, as he told director Gurinder Chadha (who directed Bend It Like Beckham) when she was casting Viceroy's House.

"I said, 'Even if I lost three stone and stuck my face in an elevator door, I wouldn't look like him'." She replied that she wasn't in the imitation game, and that Colin Firth didn't look like King George VI in The King's Speech.

The film, which comes out next year, is set in 1947, the year of Indian independence, and concerns itself with the lives of upstairs rulers and downstairs servants in the viceregal home.

"Mountbatten loved hierarchy, he loved status and medals," says Bonneville. "In our film, the pulse is what happens behind the green baize door." (This could be the beginning of a niche little earner: the daughters, the end of empire, the grand(ish) house, the lordship - can the call from casting directors looking for an actor to play former Hong Kong governor Chris Patten be far off?)

Berry, who's been checking something on her phone, now looks up and says, "And Lord Mountbatten was president of Bafta! It says here - from 1966 to 1972."

It's worth pointing out that none of Bonneville's four Bafta nominations has actually been for Downton Abbey. One was for the film Iris, in which he played a younger version of author and philosopher Iris Murdoch's husband, John Bayley, opposite Kate Winslet; and the others have been for his role as Ian Fletcher ("the sane man in a sea of fools", as he puts it) in the comedy series Twenty Twelve (a send-up of the London Olympics) and W1A (a send-up of the BBC).

Both of those TV series are far wittier and more sharply observed than anything in Downton, including Maggie Smith's one-liners. Comedy - as those who saw Bonneville's scene ("What do you do?") opposite Julia Roberts in Notting Hill will remember - is his forte. The acting in Twenty Twelve and W1A is so loose it looks improvised. It's not. Every pause, every sigh, every vocal inflexion has been scripted by the writer, John Morton. Fletcher was created "fully-formed", Bonneville says, and Morton always gives him the same two instructions: don't smile and go faster.

Next year, he's finally returning to Shakespeare. The BBC has been doing a celebrity-laden cycle of the history plays called The Hollow Crown; Bonneville will be the Duke of Gloucester in Henry VI, Part 1. (His wife, as she was in Paddington, will be Sally Hawkins.)

When Bonneville spoke to the students at Cotton Spinners Association Secondary School he'd said, about rejection, "It's hard not to take it personally - sometimes it's down to chemistry or marketability." Having haunted him in the past, the Bard's words will taste sweet in his mouth. Or, as Principal Lau put it, speaking of her Kwai Chung pupils' dreams of fame, "It is the hardships they face that make the fruits more beautiful."