Ian Hislop and Nick Newman: Burt Reynolds, the 'Private Eye' guy and the other one

If audiences at A Bunch of Amateurs find the play's plot a little far-fetched – cue an ageing Hollywood legend unable to learn his lines even when they're plastered across a fellow actor's back – they should hear what happened when a real-life celluloid crinklie played the leading role in the 2008 film version. To put it another way, if Ian Hislop and Nick Newman's first play since their Oxford undergrad days flops, the long-term writing partners should probably file for literary divorce, given the wealth of material that Burt Reynolds provided during filming.

"I think he took it a bit too literally," Newman says, recalling his joke that Reynolds should "behave extremely badly on set", as his character Jefferson Steele is a total pain. According to Hislop, by the time the Deliverance star, then in the throes of a prescription-drug addiction, had finished messing up filming there was "a black hole where there was going to be a part". The bonus is that nearly everything Reynolds did has ended up in the new script for the stage version. "It wasn't his finest hour," Hislop adds, with characteristic deadpan understatement.

Thus Steele, playing King Lear in an amateur production, breakfasts on painkillers and struggles with his lines even when being fed them through an electronic device. "Ask your editor just how much of this you want to put in before the lawyer sees it," Hislop chuckles as the pair pepper our lunch in a Soho pub with Reynolds anecdotes. My favourite: the producer David Parfitt telling his scriptwriters: "I've spent more money on Burt's wig [for Lear] than I paid you." More Hislop laughter. "So we're thinking: 'That's flattering; we know where we are in the food chain; below Burt's wig in terms of importance!'"

Not that they have done badly out of a writing career that started as teenage pupils at Ardingly College in West Sussex. Although Hislop, the Have I Got News For You team captain and Private Eye editor, is the better known – co-credits are always Hislop and Newman, never vice versa – it was Newman who paved the way, not least after Hislop followed him up to Oxford. "I'd set up a magazine with a friend of mine, then in our final year Ian turned up and took it over," Newman says. The magazine, Passing Wind, launched Hislop's career when an interviewee, the then Private Eye editor Richard Ingram, offered him a job.

Since Hislop and his wife Victoria – best-selling author of The Island – moved to Kent, the two men do most of their writing in the Eye office, with occasional forays to Pizza Express. "Essentially, we're trying to make each other laugh, and if Nick says 'That's not very funny', because I've known him since we were 15 I don't take huge offence," says Hislop. "I just wait until he tries a joke and then say, 'No, that doesn't work at all!'"

In person, Hislop is the sharper of the two but their three decades of co-credits spanning Spitting Image scripts and the recent Bafta-nominated The Wipers Times illustrate how well they combine as writers. But, celebrity being what it is, Newman tends to get air-brushed out – literally, in the case of one interview about a show they had written. Hislop explains: "They took a photo of us and then cut Nick out. All the things Nick had said they made as though I'd said them, in an 'exclusive' interview. It was almost entirely about Have I Got News, too!"

Newman is frustrated by such affronts: "I do find it annoying when I don't get credited for stuff, particularly when I'm proud of it, like The Wipers Times." Hislop chimes in: "Well don't worry, because this is going to go down as 'Michael Palin's Wipers Times'. We're both going to be cut out in favour of the actors!"

Have I Got News For You, on air since 1990 and now in its 47th series, is unavoidable, not least in backchat between the two men, as I find when listening back to the recorder I have left switched on while I ordered their lunch at the bar. I hear them discussing the previous week's show, hosted by David Mitchell with panellists Andy Hamilton and Susan Coleman; journalistic ethics probably prevent me repeating what Hislop had to say about the guests.

"That was on while we were talking. Very clever of you," he observes when I return.

He is on the record about host Jack Dee's comments to Bridget Christie, which sparked Twitter outrage for being sexist: "Oh yeah, something about how when she took off her 'No to Page Three' T-shirt the audience woke up." But he says the quip "didn't strike me as hugely offensive, because in the introduction the host is supposed to be rude. If you're not rude to the women that come on, it's like saying, 'Oh, they're not up to it.'"

He is aware of the lack of women on the show but, he says, "You can't make people do it." And its female producer, Jo Bunting, has asked "everyone you can think of" to appear. Does he stand by a previous comment, about women needing to show off and be part of gangs of blokes to make the cut? "If bantering is to be a popular comedy form, then you have to join in. You can't say, 'Oh, I don't do that stuff,' and then say, 'There aren't any women on.'" Newman adds: "You have to put your head above the parapet. You can't do it by being shy and retiring."

The only topic that quietens Hislop is gentle questioning about his family. His daughter, Emily, has just done a law conversion after graduating from Oxford; his son, William, is still there. But that's all I get. Asked if he would ever write something with Victoria, he protests, "I'm sure she'd refuse! She's got proper talent!"

Now it's up to West Berkshire's theatregoers to decide if Hislop and Newman's own strengths stretch to writing plays. "What we really want is for someone to see A Bunch of Amateurs and say, 'This would make a great film,'" Newman quips. Or perhaps I should just attribute that gag to Hislop.

'A Bunch of Amateurs' runs until Saturday 28 June at The Watermill Theatre, Bagnor, Newbury