As any fan of QI will tell you, John Sessions has a head for dates. He’s able to recall historic facts and figures with ease and often does, even going so far as to call his talent ’a sickness’.

“I have a capacity for, an obsession with, numbers,“ John tells me, in a break from rehearsals at the Hampstead Theatre. “It’s a natural thing. If you asked me to do all the kings of England, I could do it like that – all the dates, when they were born and died, and their wives.

“It’s not very useful. It’s good when I go on QI. That’s about the only time.“

It’s earned the 60-year-old actor, improviser and comedian a reputation, as he himself puts it, for being “a bit of a swot“.

“I was a very swotty child,“ admits the star of Whose Line Is It Anyway. “I used to read encyclopaedias when the other kids were out playing football. When I was young, about eight, I used to write literary biographies of famous people. I wrote about Karl Marx, William Morris, Keats, Byron. I know, terrible isn’t it.

“I’d far rather have a driving license, I’ve tried several times, I’m just no good at it. Or to be fluent in a foreign language. If I could swap all this junk and rubbish for that, I would!“

Not everything sticks in that big brain of his, however. This month, John stars in novelist William Boyd’s debut play Longing, based on two short stories by Anton Chekov, alongside Iain Glen, Tamsin Grieg and William Postlethwaite, whose father was Pete Postlethwaite. It will be his first stage appearance for some years, after his mind went blank the last time he trod the boards.

“I developed a bit of a thing about the stage, a stage fright thing,“ he says. “It came from having a terrible ’dry’ on a West End run. I had to stop the whole scene and start it again.

“I should’ve jumped straight on another horse but I didn’t. Months became years and I was doing improvised one-man shows, but the thought of getting up to do stuff I’d learnt was just freaking me out. I thought, this is silly, there’s a hole in the middle of my game. The only way round it was to confront my demons.“

John plays Mr Dolzhikov, a “self-made man, he’s vulgar, he’s a rough diamond, more rough than diamond“, in Longing, which adapts two of Anton Chekhov’s short stories, A Visit to Friends and My Life.

“It’s very modern,“ says John. “It’s a very metropolitan quandary of who are my real friends? Who are my make-do friends, my make-believe friends, the friends that don’t embody friendship?

“His plays aren’t about anything except they’re about everything. His plays are messy like life is, there’s no absolute resolution.“ It promises to be a comic tale at once exotic and familiar, with all the key Chekhovian elements audiences know and love. And there’s no danger of John choking this time.

“One can’t be complacent. You feel a real onus on you to be up to standard and to do your best.

“I learnt the whole play, before we started rehearsing I knew all my lines. I was determined to nail it and be absolutely on top of it.“

  • Longing is at The Hampstead Theatre, Eton Avenue from February 28 to April 13. Details: 020 7722 9301, www.hampsteadtheatre.com