I went to Cardinal Manning Roman Catholic School near Ladbroke Grove in west London. As a secondary modern it was probably the Cinderella school in the area but there was one teacher there who had a huge influence on me.

Martyn Corbett was my English master all the way through school. When I told him I wanted to be an actor he didn’t pat me on the head and tell me I would grow out of it. He was the first person in my life who took it seriously. He said if I was serious, I had to be serious; I would have to work hard for it.

He started up a drama club with an end-of-school play and it was during these sessions that I came alive. I was a geeky kid at school. I wasn’t sporty or popular - I was tall for my age and gangly and clumsy.

If there was ever a party, all the cool kids would be in the kitchen getting off with each other, and I would be the one in the corner of the living room, putting on records. I never got laid, but I could name the bass guitarist for every band you care to mention.

Things were different in drama club, though. Martyn would take us through improvisation games, vocal exercises and texts. I was always first in line for drama club, always on time.

We would perform plays such as The Taming of the Shrew, The Caucasian Chalk Circle and Murder in the Cathedral. Even though it was just a secondary modern, it did have pretty good facilities, and he deliberately set the bar very high. He wanted the plays to have value and contribute to our overall development.

On a day-to-day basis I wasn’t a failure, but I was a nondescript pupil. I was never top or bottom of the class - I just scraped by. But I lived for those school plays. For once I really felt I was in my element.

Martyn suggested I audition for the National Youth Theatre. He coached me and I got in.

He has retired but we are still friends. He lives in Spain, but we email and write to each other regularly and he has travelled all over the world to see me in plays. I am never more nervous than I am when he is in the audience. It didn’t really bother me what my parents thought of the performance, but I really wanted Martyn’s approval.

Years later, I was in a play in the West End and Martyn joined me and some friends for dinner afterwards. Someone asked him what I was like at school and he said: “He was a dreadful actor, but he was a wonderful show-off and he had no fear.”

If it wasn’t for him, acting would either have remained a far-fetched dream or it would have worked out in the end but without the same preparation and insight that Martyn lent me.

I first expressed a desire to be an actor at the age of nine, apparently, but there was no guarantee that I would succeed. Most of the boys at school said they wanted to become priests. But Martyn believed I could do it.

He was wonderful, but he never blew smoke up my arse, as they say in the States. He laid bare all the possible pitfalls and obstacles. He never sugar-coated things: he said it was likely that things would not work out with acting, but he never put me off. He energised me. He gave me books to read and the tools I would need to get by. I have never forgotten that.

In a way, he was like a surrogate dad. My mother had dreams of being an actress but my father dismissed the idea of acting. He said I must either be a degenerate or out of my mind. He thought I would grow out of it.

Even when he saw me working, he never quite acknowledged it. At best, he thought I was lucky. Thank God for Martyn. I lapped up his advice. His commitment gave me a willingness to keep going.

I never shirked from the work he gave us. The experience has led me to try my hand at teaching at the American Academy of Dramatic Arts in Los Angeles.

I realise how important a good, caring teacher can be. You handle young people’s dreams carefully because they are easily shattered, and that can have a terrible effect on a person. If it doesn’t work out, it doesn’t work out, but they need to find that out for themselves.

I will always be grateful to Martyn for taking me and my desires seriously.

Alfred Molina has starred in plays and films, including ‘Chocolat’, ‘Frida’ and ‘The Da Vinci Code’. His latest film, ‘An Education’, opens today. He was speaking to Hannah Frankel.