Lee's research covered medical reports on the internet, books he ordered from Hong Kong and the US, and conversations with doctors, nurses and social workers.

In his film, a schizophrenic man (Lau) convicted of involuntary manslaughter is released by his doctor (Huang Xiaoming) from a psychiatric facility three years later. The decision brings intense pressure from the medical staff, family and society at large.

"When I read Lee's script, the thing I noticed most was its focus on discrimination and the pressure of living in the city," says Yee, who co-produced the film with his wife, Mandy Law Hiu-man, and veteran filmmaker Lo Chi-leung.

This is not the first time the pair have collaborated. Lee worked with Yee as an assistant director on the 2010 crime thriller , after directing the 2008 youth-oriented haunted house drama .

A Cornell University graduate who went on to study film at City University, Lee worked at Tsui Hark's Film Workshop in 2001 and served as assistant director to Andrew Lau Wai-keung on high-profile movies such as (2003), (2003) and (2005).

Mental illness is a familiar topic to Yee; his own directorial debut, (1986) - which earned him nominations for best picture, director and screenplay at the Hong Kong Film Awards (on top of best supporting actor for Paul Chun Pui and a best art direction win for Yank Wong Yan-kwai) - was a harrowing look at the lives of sufferers in an uncaring society.

In producing , Yee thought the time was ripe to take advantage of the financial flexibility afforded by a Hong Kong-mainland co-production. Something working in the project's favour was that, in the three years he's been making films on the mainland, Yee has noticed audiences' shift in interest from historical dramas to contemporary stories.

"Even Peter Chan Ho-sun made [the present-day drama] ," he says. "When I took the Beijing subway, I could see the stress on people's faces - even more so than in Hong Kong or Japan. Mental conditions are still a new topic for movies in China, and few directors have proved adept at handling it."

And neither have the mainland censors. While Lee concedes that he's "not experienced enough to talk about the adjustments required of co-productions", Lo downplays failure to pass censors on its first try as a mere technicality. "I don't think there was any problem with the script. It may have more to do with the words we used to tell the story. With different wordings, we could have passed the first time," he says.

Yee says there shouldn't be a problem with censors as long as the story is based on science. "You can't just randomly invent a new disease," he says. "It's understandable for the mainland authorities to be careful, because there isn't a film categorisation system [on the mainland] yet. As filmmakers, we have our responsibilities because some of our viewers may be less mature in mind."

When asked if he deems a realistic portrayal of the mental issues he studied, Lee doesn't directly answer. "Reality these days is more dramatic than fiction," he says. "Lo and Yee told me I should refrain from making a sensationalistic film. I don't think of myself as the saviour to eradicate discrimination from the world. That was not my point."

In a similarly charitable spirit, Yee denies going out of his way to give Lee's career a helping hand by producing his film. "This feels most natural to me," he says. "The fact I'm working in this field is precisely the result of all those opportunities offered me by others at the beginning. Now I have reached this age, it's what I should do.

"If I produce a youth romance by a mainland director now, it might be partly because I wasn't going to direct such a film anyway; those projects belong to my past. There are films that are beyond my energy level. If you asked me to shoot [a hit crime thriller Yee made in 2004] in 28 days all over again, I might just give it over to someone else."

Yee has been giving plenty of opportunities to others. Since he became involved in plans for a boutique diving resort with several friends two years ago (which still hasn't opened due to his schedule), he has worked non-stop and produced (2014), (in cinemas now) and as well as upcoming projects by Lo () and Cheung King-wai ().

With Yee directing two films of his own - the 3D martial arts drama and , a drama about film extras that's "a gift to myself on my 40 years in the business" - it sounds oddly convincing when Lo jokes that he was enlisted to co-produce because Yee was just too busy.

"I'll organise an anniversary party on the night of the Hong Kong Film Awards, on April 19," says Yee. Although he hasn't taken a day off for more than two years he stops short of saying that he's going to ease off soon. "I'll make an announcement about my future then. I'll keep that a secret for now. "

At the other end of the career curve, Lee's focus is strictly on the present. "After watching , I just hope my audiences will care more about their own mental well-being and that of the people around them. I hope they'll be more tolerant of themselves and others.

"Society today is too angry ... Compared with the time I was growing up, the population seems to have grown much more distant. Maybe everybody is facing huge pressure nowadays. So don't forget to be nicer to yourself."

That's advice his producer can probably learn from.

opens on April 2