Your Orson Welles biography is an impressive and colossal undertaking with volume three (the penultimate book) just out. But how would you explain Welles’s claim to immortality in a sentence?
Orson Welles approached film with the ardour of a new lover. He explored its possibilities with an almost adolescent excitement that is contained in his films. That is why seeing Citizen Kane makes people want to become film directors and why Welles will live for ever.

He was larger than life, wasn’t he? Would it be fair to say you don’t do anything by halves either?
I bite off more than I can chew and end up chewing very hard indeed. I started this damn thing in 1989. Who could have imagined it would take a quarter of a century? I disapprove of long books, you know, but Orson was that kind of a guy.

No shortcuts possible; are you in any way similar to him?
We’re both autodidacts. It is a critical thing, it separates people. And I am an omnivorous reader. Also, Orson always said – and I relate to this – that he was essentially lazy. I am too. It may be that it is the lazy people who have to keep whipping themselves into actually doing something and become the biggest achievers. I don’t know.

I’m amazed because energy is the quality I associate with you
I don’t think you can separate energy from greed. I’ve just written a biography of Wagner. I look upon these projects as great big banquets. They’re irresistible – I gorge on them. Then I realise: oh my God, I have got to write a book…

Would you and Orson have got on?
I’d have fallen under his spell because almost everyone did, as long as they accepted him on his own terms. Whether he’d have liked me is more problematic.

As an interviewee, Welles was almost too good. He had a lot of front. How good are you at managing your own image as an actor?
Sometimes people do want me to behave in a certain way. Sometimes one lives up to it, sometimes one refuses. Everybody wants me to be like Gareth from Four Weddings and a Funeral.

You once said you were a person without secrets – partly because you have always been open about being gay – but are you really secret-less?
I’m very transparent, too transparent for my own good. I’ve always felt that the great actors have mystery. Unfortunately, I have none. I wish I had, I really do. John Gielgud did not have mystery but there was something lurking inside that was ineffable, unfathomable.

Where does your love of biography come from?
I came from a Catholic family and read Butler’s Lives of the Saints – a 12-volume book. I can probably trace the influence of that in the biographies I have written in that they are exemplary lives. One is always looking for the meaning of a life. Hard with Welles. Easier with Wagner and Dickens. I fell in love, as a small boy, with classical music. I read biographies of composers. They were written for children and completely sanitised: Tchaikovsky as lonely bachelor, Mozart as sweet, periwigged boy. But I adored them. I’ve always been interested in other people’s lives, what they make of the cards fate has dealt them.

Perhaps acting is a form of biography?
It is, very much. And even in acting, I’ve been drawn to huge characters. A character like Falstaff is without boundaries and lollops around inside the play destroying its structure. Intemperateness has always interested me. I was an intemperate child: incredibly demanding, a nonstop all-singing, all-dancing event in people’s lives, to an exhausting degree. I understand that in others.

How much is acting about living in the moment?
Living in the moment is the hardest thing. As an actor, each night, you have to forget you know your own lines so that it is as if what you say you have never said before. Some actors regard this as a burden. I regard it as the blessing of theatre: I get another chance to get it right every single night.

You have to be your own critic – how does that work?
You are constantly monitoring a performance. It is like the patient and you are the doctor. You might think: ah – this is a very good development, the pulse is quicker now… I try to open myself as much as possible during a performance to intuition, so I am in time and in touch with the audience and so we are breathing the same breath and our heartbeat is at the same level. That is when things begin to happen.

How good are you at doing nothing?
Pretty useless, but I walk the dog. I had twin boxers, Biff and Bloxie, but Bloxie died earlier this year in the sweetest most gracious way – she just slipped away. Now I just have dear old Biff who is a funny old chap, a wonderful creature. He is 12, which is old for a boxer. He had a stroke about eight weeks ago, which was terribly scary because it resembled the stroke his sister had, but he pulled himself right round. He is such a repository of love, affection and hilarious reminiscences. Like all boxers, he is a character.

If life is not going according to plan, what do you think the key thing to remember is?
Live in the moment and achieve something. Even if it is only an essay. Plant something. Cook a souffle. Anything, so you can say: well, I did that. I can still do that. And a good souffle is not to be sniffed at.

And on the subject of cooking as we accelerate toward Christmas, are you any good at it?
I started very late. I’m very much an adventurer as a cook. It is like Russian roulette, eating with me. Christmas is different. My partner, Seb, and I will spend it with his sister and their kids. In a very old-fashioned way, I do like to connect with kids, particularly at Christmas. So Seb will cook and I might contribute some modest thing like a soup. I’m rather good at soup.

What is coming up on the acting horizon?
I’m rather excited that next year I’m playing the lead in a sitcom, The Rebel. It is based on a column in the Oldie about an extremely angry 66-year-old man (I am 66). He is like Victor Meldrew on steroids. It is very funny. We shoot in Feb, for UK Gold.

Simon Callow’s book Orson Welles, Volume 3: One-Man Band (Jonathan Cape, £25) is out now.

Click here to order a copy for £20