A caper is a criminal enterprise- usually theft, burglary or robbery- often portrayed, in books and films, with humorous overtones. To his readers and crime-writing colleagues, Donald E. Westlake is the undisputed King of Comic Capers. Richard Stark, by contrast, writes bleak, hard-boiled novels about a nasty professional criminal. It's no secret that the two award-winning authors are the same person.

Westlake (his real name) has had other pseudonyms, too. (“Publishers don't like to publish more than one book a year by the same author, and I was writing faster than that.”) He has written more than 90 books, some 20 of them being filmed, which makes him probably the most filmed living author. As a screenwriter, he has picked up an Academy Award nomination for his adaptation of Jim Thomson's novel The Grifters, directed by Stephen Frears.

In the flesh, this crime-writing phenomenon turns out to be a jovial 74-year-old New Yorker full of laughter, showing no signs of authorial angst and seemingly untouched by overweening ambition. He is proud of his professionalism and of his success. For more than four decades he has amused, entertained and occasionally shocked large numbers of readers. Here, I thought, is a writer satisfied with his lot.

Westlake's two main characters are both career crooks, with nothing else in common. John Dortmunder — featured in 13 novels — is a brilliant master criminal; imaginative, even artistic in his work. Unfortunately, he's also supremely unlucky. However cleverly prepared, his schemes founder, the victim of malign chance or the stupidity of his associates. In What's So Funny?, his latest escapade, a former cop blackmails Dortmunder into attempting to steal a chess set made of gold and jewels. Two problems. It weighs 700lb and is kept in an impregnable vault under a New York office block. The book is fresh, funny and inventive; Westlake is as sharp and sparkling as ever.

Parker, the anti-hero of 23 Richard Stark books, has no first name, even in Westlake's mind. This is noir of the highest order — indeed writing as Stark, Westlake is perhaps the last of the old-school noir writers. Parker is pitiless, cold and, if necessary, violent. He lacks compassion. In the film Point Blank — from Stark's novel The Hunter — he was played by Lee Marvin at his most menacing.

The Parker novels suddenly stopped after the sixteenth, in 1974, then resumed, with equal lack of explanation, more than 20 years later. The missing years have long puzzled fans. “I didn't know the reason then, but I know now. I didn't want to stop writing about him. I tried the next book three different times, but they all ground down. Now I see what happened. When I first came to New York, no one in my family had ever been involved in any of the arts at all. I didn't know anyone in the publishing world, I hadn't been to any of the right schools. I was a barbarian at the gates. The first Parker novel begins with him walking into New York and creating an identity for himself; saying, goddam it, here I am. That was me. That was 1961. But by 1974 I was successful, making a living, I'd sold movie rights for my books, I had a bit of a reputation, and it was very hard to keep that ‘outsider' muscle.”

The next Parker wasn't finished until 1997. Why did he return? The answer is unexpected. Westlake sees it as a kind of sucks-to-you, one-finger gesture to Hollywood. He had twice lost screenwriting jobs (one of them an Elmore Leonard novel) because the producers said that he was too old (he was in his sixties). “Parker came back to say: ‘I'm older than you but I'm still smarter than you. I'm better than you, faster than you and I'm still prettier than you.'”

Why hasn't Parker got a first name? “At first, he wasn't supposed to be a series, so I didn't bother. Then I kept it that way. If I'd known it was going to be a series, I wouldn't have called him Parker. For more than 20 books, I've had to find some other way to say ‘Parker parked the car'.”

But how can you write about someone over such a long time, and understand him, without knowing his name? “Aloofness is part of his character. He's aloof from me as well, which makes it possible for me not to know his name.”

Writing about John Dortmunder and his hapless associates, Westlake says, is “like hanging out with old friends. He's a fatalist, as I am. I don't really expect things to work out. But I'll survive. I'll get by.” There's no such warmth in his relationship with Parker. “With him, it's like having someone doing carpentry in your house who doesn't talk. He's very competent but he's not fun.”

Westlake is slowing down a little: “I spend more time not knowing what happened next.” He has never planned his novels. “ I don't do anything except gradually tell myself a story. So every day it's ‘now what?' I don't know what my character is going to be doing next. I almost never know the ending, and I certainly don't know anything in between. It gives me great pleasure to tell myself the story.”

Westlake still writes seven days a week, taking his cue from the violinist Yehudi Menuhin. “I once asked him if he practised every day. ‘Yes,' he replied, ‘if I miss one day, I notice it; if I miss two days, the conductor notices; if I miss three days, the audience notices.' You have to keep going.”

What's so Funny? By Donald E. Westlake

Quercus, £12.99

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