Jack Dee has a certain way with people who come up to him on the street. “If you’re grumpy with them, they think they’ve had their money’s worth. I don’t mean to be grumpy, but I don’t necessarily smile. They say: ‘I like your show’; and I say: ‘Thanks very much’; and they say: ‘You’re so funny because you don’t smile.’ Then they go away happy because I didn’t smile.”

Actually, Dee, 42, is quite approachable. And faintly scruffy, in an open-necked shirt and creased trousers. What’s going on? I’m being presented with a smiley Jack Dee in informal men’s wear. Dee acknowledges that the smartly dressed curmudgeonliness which cements his act is but a facet of the Dee character. “It’s like if you had met, say, Tommy Cooper. You would be surprised if he was exactly like he was on stage but disappointed if he was nothing like he was on stage. What you get in his comedy is a strand of his character, blown up. That’s what most stand-ups are doing. The successful ones, anyway.”

It may be a “strand” but pessimism is big with Dee, and he has to be careful not to bring it home with him. “Tony Hancock was unable to stop the character on stage overtaking the character in real life. And it was catastrophic. I’m sure that could happen to me; I think depression is my default setting. It’s something I have to be disciplined with, and not totally wallow in. You can pull yourself up, you know.”

He and his wife Jane have four children aged from 13 to 7; is he concerned that they might inherit the Dee gene for melancholy? “I hope they haven’t and I don’t think they have, not yet anyway, because they are all so jolly.”

So how does the unjolly member of the Dee household tackle it? “If I’m not busy, I can get low. I need to be physically busy and have lots going on. I don’t have a great concentration span but I need to go and see films regularly, I like to have a good book on the go. I don’t like to be on a car journey without a newspaper. I like to have some input, all the time. I don’t like the idea of wasting time.”

In any sphere. “It means having a dictionary in the lavatory so that when you are on the loo, you can look up a new word. At least you haven’t wasted the time when you’re having a crap.” That’s crazy, I say. “Is it? I read somewhere that in terms of study hours, the time you spent on the loo you could have worked your way to a degree. You have to make the best of every moment.”

These lavatory-associated dictionary habits perhaps spring from a sense of educational want. A thoughtful, serious person, Dee did not arrive at his position of celebrated comedian via the somewhat hackneyed route taken by those over-confident public school types who found they were funny in the Footlights.

Educated in the state system, Dee doesn’t have a degree. He grew up in Kent, left school after bombing his A levels and entered the catering trade at 18. He earned his stand-up colours at the Comedy Store, the London venue famous in the 1980s for its “alternative” comedians. Here, he found he got laughs by being outrageously rude about people. Fat people, for example. “That was controversial because you aren’t supposed to say these things. Now, I have to look more carefully because I’m in an empowered position of celebrity. Whereas saying those things as an unknown grumpy little f*** who’s just got out of the audience, I think in that context was very funny.”

His act is now a perfectly honed repertoire making reference to the many things that annoy him. For example, when you go to a dinner party and your host insists you take your shoes off “so you have to pad around like a mental patient. The attractive thing is to pinpoint something that does annoy me and to discover it annoys everyone else, too. The human spirit is a remarkable thing. It can climb mountains but stumbles on gravel. People go through the most unbelievable traumas and cope and then go completely ape-shit when they get a parking ticket. It is a cliché but, when someone is ill, people say it puts things into perspective. And I think yeah. But only for a bit. Because it’s so quickly that you get back to being annoyed that you couldn’t get the shopping trolleys separated at the supermarket.”

I’m laughing, Dee is chortling, and it all seems very easy. Not so. “It’s taken me a long time to come to terms with the anxiety involved in performing,” he says. “I feel very, very low before a show. The crucial time is the 15 minutes before I go on stage. In that time, some little light goes on, and I think about ways of starting the show.” So, it goes thus. Five hundred people are sitting in some theatre, waiting for Jack Dee to come on stage and crack them up. Meanwhile, Dee is going through hell backstage working out the script.

“I don’t like just going on and reciting my first gag. I have to find some comment about the town or the theatre. And that’s what gets the juices flowing. Then, during that walk to the microphone, the light comes on. The terror is that it won’t. People ask me: ‘Do you ever die on stage?’ Well, yes, I do. The audience isn’t aware but, internally, there’s nothing creative happening. Those are the worst moments.”

Jack Dee sets a lot of store by being an accountable, honest person, which is probably why advertising companies love him so much. It’s also probably why he won the first Celebrity Big Brother, where Dee performed everyone else off the screen by simply refusing to show off. “What surprised me about the other people in that room was that none of them appeared to have any hinterland,” he says. “It just seemed that I had more of a grasp on reality. They were amazed that I knew how to make bread or had dogs and walked them. They seemed to be nothing other than what they did professionally.” And if what you do isn’t very much, frankly, then please don’t ask for Jack Dee’s approval — because you won’t get it.

“To me, one of the mysteries of the modern age is that people want to become famous at any price, and appear unperturbed by the fact they have no talent to offer, no gift to share with the world. They are just thrilled to be photographed at a preview and that’s enough. I would be terrified if that was my lot. To me it’s all a bit darker than that. That’s why I find the whole culture of celebrity shows so disturbing.”

He’s a contradictory fellow, Mr Dee. A man who hates the trappings of celebrity but won Celebrity Big Brother; a self-confessed alcoholic who joined AA but still enjoys a drink; a Christian who isn’t a churchgoer. How does he work himself out? “Well, I don’t think being contradictory is unique. I am not a churchgoer. I don’t associate my beliefs with any formal body. Most of what I see as being portrayed as Christianity is absurd anyway. I find it difficult to square anything with the evangelical Right.”

His belief system, if you can call it that, is much nearer at hand. “For me, comedy is the thing that makes sense of life and makes sense of my life. It’s like having a faith. Comedy makes sense of things that people can’t understand, or can’t come to terms with. When someone is honest about life, they laugh because they know it to be true.”

You believe in comedy but the Dee outlook is hardly sunny. Frankly, if we were to step outside his office in Soho and ask everyone whether Jack Dee’s glass was half-full or half-empty, we know what the response would be.

“I’m not a pessimist, I’m an optimist,” he protests. “I’m the biggest optimist I know! My glass is not half-full, it’s completely full. It’s only when I get to it that it turns out to be half-full. I never give up on the hope that things are going to be great. But the trouble is that they rarely are.”