She decides to kidnap a rich child, but grabs the wrong one. He turns out to be the son of a divorced inspector played by Renee Liu Ruo-ying. A cat-and-mouse chase ensues.

Playing a disturbed woman who moves seamlessly between charming, calculating, evil and sweet was a challenge for Lam, but one she relished.

'She wears many masks,' said Lam.

'When she is with her husband, she plays one role. When she faces her close friend [a police inspector played by Cheung Siu-fai], she plays another. She lives a lie.

'It's a natural shift for the drama instructor to become the kidnapper because she thinks of the whole situation as a plot or scene in a play. She fabricates an imaginary world and doesn't realise that the kidnap is for real until she's exposed by Cheung Siu-fai's character near the end of the film.'

Lam's comment perhaps sums up acting, which is the art of blending reality with fantasy until you can no longer tell the difference.

Early in the film, there's a scene in which Lam's character throws a shoe and barks at a performer who keeps repeating the same mistake during a rehearsal.

This is a scenario that Lam witnessed first-hand when rehearsing with her drama teacher, the renowned comedian and stage artist Jim Chim Shui-man.

'I thought about Jim while shooting that scene. Once Jim got very annoyed because an actor was making the same mistake over and again. He suddenly yelled at the top of his voice: 'Stop!' It was quite shocking.'

Lam has been taking acting classes with Chim for a few years. This is rare for an actress of her calibre. After all, Lam won two coveted acting awards - best supporting actress and best new performer - at the Hong Kong Film Awards with her stunning debut as a precocious schoolgirl in July Rhapsody five years ago.

'I decided to learn acting with Jim because I felt I lacked something, such as a spark, inspiration or acting skill,' said Lam.

'My acting had been based on my intuition, my knowledge of the characters and my personal experiences. I felt it was time for a change.'

The same drive compels Lam, despite her girl-next-door looks, to take on roles that would deter many local actresses.

She plays a deformed lunatic in the horror film Home Sweet Home and a disturbed woman in another one of Lo's psychological thrillers, Koma.

'My career could have been easier. I could have picked simpler roles. But that's not what I wanted. I would rather accept roles that nobody wants, or roles that I'm not sure if I could handle,' said Lam.

'I have no bottom line when it comes to acting.'

Lam has been in the film industry for more than five years, but considers herself a beginner. Looking up to the likes of Juliette Binoche, Issabelle Huppert and Anna Karina, Lam said her 'life onscreen has just begun'.

Each new film is a new world, a new experience.

'I've learned to listen to my body and understand myself. I used to think that acting was about memorising the lines and doing what the director tells you,' said Lam.

'Now I know there's a lot more to it.'

Kidnap is now showing.