'It used to be called a mixed throw and I broke too many ribs. Didn't mean to, I'm not interested in sadism at all. Or masochism.' Tony Slattery is revealing all about his short-lived judo career as a 15-year-old. 'I was a black belt and represented England once, in Rotterdam against the Netherlands. But the throw was deemed illegal and I didn't enjoy breaking ribs. So I was disqualified.'

The Whose Line Is It Anyway? legend would be the first to admit that he's spent a considerable amount of time on life's canvas and is still grappling to an extent. Our conversation has been delayed a few hours because 'a cat pissed' on his phone. It's unclear whether this was his cat or a random act of feline malice. Or if his ribs were broken or an adolescent Dutchman's. Notwithstanding his well-documented mental-health struggles and erstwhile drink and drugs excess, Slattery retains his twinklingly impish sense of humour, a compelling mix of wounded pride, disarming vulnerability and gushing graciousness.

Recently reacquainted with footage of his audition for the title role in the 1996 Doctor Who movie, in which he offers a cheeky, proto-David Tennant turn, he faux-bitches, 'I was second choice because Paul McGann got it. And it was a really horrible reason why he got it. Because he was better. And I'll never forgive him for that!'

Returning to the Fringe for a third year in a row, the 59-year-old is reuniting with some of his fellow Whose Line? luminaries, 'Clive and um [he clears his throat], no, I'm sorry, all the other names have disappeared…'; performing a few ad-hoc improv shows on the Free Fringe with his regular collaborator Allan Lear ('I'm sure people will show their level of enjoyment in the bucket at the end'); and being interviewed about his life and career at The Stand by comedy historian Robert Ross in Slattery Will Get You Nowhere. 'The agreement is that I don't know what he's going to ask me and what clips he's going to show. And I don't know what I'm going to say. If it doesn't sound too pretentious, and I'm sure it will, I'll open up a bit and he'll show all the stuff I've done in my worthless life.'

That's not true. One of his most treasured memories is Stevie Wonder telling him he had a kind face. 'I wasn't sucking up to him,' Slattery stresses. 'But I said to him, "Mr Wonder. You've been blind since birth, but because of your articulacy and songwriting, how would you describe the colour red?" And we had this really in-depth conversation. None of it was used unfortunately because it was for Saturday morning kids' TV. Still, on my gravestone, if I have a gravestone, that's going on it: "Stevie Wonder Felt My Face!"'

He also speaks fondly of his festival debut in 1979, with Stephen Fry and Rev star Simon McBurney ('when we were young, innocent and less fat'), two years before he scooped the inaugural Perrier Award with the fresh-faced Cambridge Footlights' line-up of Fry, Emma Thompson, Hugh Laurie, Penny Dwyer and Paul Shearer. He ponders if it's 'gratifying that people are still interested in a palaeontological old fossil like me? Yes, yes it is gratifying. I'm really looking forward to it; it's one of the most beautiful cities in the world. And I promise to behave myself.'

Tony Slattery: Slattery Will Get You Nowhere, The Stand 3, 13–25 Aug, 3pm, £12 (£10).