But then I got this terrible skin problem, psoriasis. Think Michael Gambon in The Singing Detective. It was itchy and flaky and it bled. It was very unpleasant-looking and very painful. In fact, it's a condition that imprisons you. In those days the only treatment was to go into hospital and be wrapped up in coal tar, which I did. Then as soon as I came out I'd worry about when it would hit again.

Psoriasis is hereditary. My father had had it very badly and I saw how people reacted to him — they couldn't bear to be near him. It was like a stigma. At lunches I wouldn't wear a dark jacket because I would leave a trail of skin stuff behind. Staying in hotels on location, I'd wake up in a mound of dead skin, and I'd have to try and scoop it up or explain it to the chambermaid. And I'd be reluctant to take my shirt off to play love scenes.

The psoriasis gave me terrible complexes. It made me withdraw totally into myself. It affected my career and my personal life. Trying to get girlfriends, it made me feel I wasn't attractive. I felt I didn't want to have a child if they were going to inherit the condition. It made me terribly worried about my work. I drank a bit too much to compensate. And I couldn't talk about it except with close friends. It was a pretty grim time.

Perhaps not getting some roles was just part of the whole acting business, but sometimes I couldn't work, couldn't face going up for a part. I don't really blame any professional failures on the psoriasis, though. I still had it in the early 1970s when I did the play of I, Claudius. It wasn't particularly successful, but it was then that I started to go to the theatre again. I quickly realised I was feeling very uncomfortable watching actors on stage. I began to think: "How do they do it? How do they learn the lines? How the heck can they stand in front of all those people?" It was like looking through the opposite end of a telescope with the big end pointed at me. It was the beginning of stage fright, a very weird time.

Once, at a play, I perspired so much watching the actors that I had to leave. "How on earth can they bear to be on stage?" I wondered. I knew I certainly could no longer go on stage myself. The psoriasis was at its worst when I did the film The Omen, in which I played Gregory Peck's sidekick. Gregory Peck was such a lovely man, and funnier than you might think. He was so understanding. So the condition also revealed how fantastic some people could be.

During the stage-fright years, I was fighting two things. First, I didn't want anyone to know I had stage fright. It also meant I had to do work I might not otherwise have done. I was married to an American [Sheila Kent] at the time, and I went to live in America. People thought I'd given up the stage and gone all Hollywood. But the truth was, I thought that if I couldn't do theatre, I might as well earn my living in films and TV. I could also go to hospital when I needed to, and I didn't have to worry about appearing on stage. I didn't talk to anyone about it — I was in denial, I suppose. In America I met some fantastic people making films. But going there felt like a bit of a defeat. I mean, why should I leave my country to work? But there was far more TV and film in America then. At the time I didn't link the psoriasis with the stage fright, but it may have made me more sensitive and introverted, which might have led to it.

I did no theatre at all for 29 years. But then in 2001 I was asked to play in Shaw's Major Barbara on Broadway. By then I was on various medications which hadn't previously been available. They're not a cure for psoriasis but they control it. So it didn't occur to me to be nervous.

Unfortunately, four weeks after the play opened, the planes crashed into the Twin Towers. We closed for a couple of nights, then carried on. The audiences weren't huge. But I'd done it — gone on stage in a leading role — and I hadn't suffered from stage fright. It was a joy to be back.

I've only done 11 plays in 40 years, so I can hardly call myself a stage actor. I've missed a quarter of a century of theatre. There are many parts I'm too old for now, and that's a regret. One of the reasons it's great to be playing Lear is that at one point in the play he takes his clothes off. And now I can actually do that — take my clothes off without feeling worried. In fact, I could lose my trousers in a farce if I wanted to. That could really be fun."