There’s a Northern Irish actor, a Welsh comedian and an English writer in a bar. Not the first line of a racially suspect joke, but an accurate description of my meeting with the West End’s new theatrical supergroup.

Kenneth Branagh (most recently the director of the blockbuster movie 'Cinderella'), Rob Brydon ('Marion and Geoff', 'Gavin & Stacey', 'The Trip', Branagh’s 'Cinderella') and Sean Foley (the splendid Morecambe and Wise tribute 'The Play What I Wrote', which Branagh directed) are seated on velveteen chairs in the beautifully restored gilt-walled bar of the Garrick Theatre on Charing Cross Road.

Foley’s version of Francis Veber’s French farce 'The Painkille', which he is also directing, is about to play here, starring Branagh and Brydon as professional killer and suicidal press photographer respectively.

The action takes place in two adjoining hotel rooms that overlook a courthouse. One is occupied by Brydon, a depressed, cuckolded Welsh local press photographer, the other by Branagh, a smart, thin-lipped hitman who has killed a witness in a current trial before he has a chance to name names. Quite something to put on and pull off in Belfast, Branagh’s home town, where it opened in 2011, although it’s hard not to like the theatrical gymnastics, lightning banter and emotional end-note.

The drama is part of Branagh’s impressive season at the Garrick, which brings together seven plays cast with favourites such as Judi Dench, Zoe Wanamaker, Adrian Lester, Lily James, Richard Madden and Derek Jacobi. The first in the season, 'The Winter’s Tale', with Dench, won mostly rave reviews and was a sell-out.

My meeting with the trio feels as though it owes a little to the laws of farce — although, thankfully, no one tries to exchange clothes, hide in a cupboard or leave through the window. Brydon, 50, arrives first. We talk about a thumb injury that he received during a preview performance. It left him frantically working out while on stage how he was going to get through the rest of the physically demanding show. In a dark blue suit and white shirt with open collar, he’s relaxed, naturally funny, immensely likeable and clearly enjoying the gig.

Branagh, 55, is next in. The actor, who was knighted in 2012, once said that “my definition of success is control” and this rule will be applied for the next 45 minutes as I struggle to get him to answer any questions that aren’t his or Brydon’s. When Foley, 51, arrives to complete the threesome, it becomes clear that the younger men take Branagh’s lead — although, thankfully Brydon allows his reverence towards Branagh to include some gentle teasing. Branagh, in the smart-casual uniform of dark blue jeans, jacket and woollen scarf, tied French-style, shakes my hand. “Thanks for talking to us. I’m glad you’re interested,” he says.

If this bunch were a boy band Branagh would be the charismatic, intense lead singer, Brydon the funny, laid-back bass guitarist who gets the girls and Foley the bookish synth player who never misses an opportunity to quote his cultural heroes.

We begin with the script. I’ve just finished reading the latest version. It’s huge fun and whizzes along. “Oh forget all that, it’s all changed,” Branagh barks. “You think we’re joking.” There have been nips, tucks and some interesting enlargements, he says, since Belfast. They have improved how the two main characters interact and bulked out the part of Michelle (played by Claudie Blakley), the wife of Brydon’s character, Brian Dudley. “That was where we put the writing back into it,” Branagh says.

Presumably you were all involved in the rewrites? Foley laughs. “Unfortunately, yes.” Branagh chips in: “No, brilliantly, yes.” He smiles. “That is where the collective comic brain of us all has got involved.”

The collaboration between Branagh and Foley began with the Right Size, the theatre company that Foley founded with Hamish McColl in 1988. This project came about when Foley watched Veber’s play in Montreal, Canada, in French. He didn’t know much about it, “but he recognised a fantastic set up when he saw one”, says Branagh.

Foley picks up the tale. “I got in touch with Frank [Francis Veber] and realised that there had been no English-language version. So I wrote this and sent it to Ken and asked: ‘Will you do it?’ ” Branagh continues: “We got a reading together and that’s when Rob came in the studio theatre in Hampstead five or six years ago. We had a crazy day.”

Brydon sighs. “At home I had imagined Ken and myself and thought, ‘That’s interesting casting’, and when we met we hit it off. It was the first play I had done outside of college and school so it was a great start.”

There is jeopardy every night. At the performance the evening before our meeting Branagh threw a gun into the audience by mistake. Luckily it didn’t knock anyone out but you can imagine what went through his mind: an evacuated venue, injury, lawsuits. Branagh relives the experience. “I threw something up in the air and saw this object head to someone in the front. It bounced and went to the side. My brain was like, ‘S***, call an ambulance,’ and then it landed. It will never happen again. Code red was flashing.”

Brydon jumps in: “Often the audience is not aware because, as an actor,” he puts on an English stage voice “And I have GREAT experience as an actor . . .”

Branagh interjects: “Because this is number four [play] isn’t it?” he says with a girlish smile (as well as the Belfast version of this play Brydon has starred in Trevor Nunn’s production of 'A Chorus of Disapproval' and Matthew Warchus’s 'Future Conditional' at the Old Vic). Brydon: “Yes, when you have done as much theatre as I have!” He giggles. “I am offering Ken so much advice but he doesn’t seem to want to take it.”

Branagh draws a halt to the post-joke laughter that Brydon has produced — with a compliment. “He [Brydon] underestimates his phenomenal understanding of what can be funny for a large group of people and his incredible experience — and Sean too,” he says.

There are some explicit, some might say crude, scenes in the play. Were Belfast audiences offended? Foley looks to Branagh but then responds himself: “I do remember an old mucker of yours turning up and saying that five years ago you couldn’t do something like this in this town. The fun of farce is that you see the characters caught in a knot of disaster.”

So is farce harder to do than tragedy, I ask Branagh, who has, of course, great experience of both from his early days at the RSC and beyond. Branagh pauses and answers a different question — his own — about his season. “'The Winter’s Tale' proved that these shows are getting opened up into an audience that is very much more connected and demanding; no one sits back in this theatre, they lean forward.”

Brydon, perhaps sensing my frustration, begins to interview Branagh himself. “I’m interested,” he looks at Branagh. “So what other things have you done?”

Branagh answers him straight. “I’ve done a film this year and comedies in the theatre, and if I may say so, it is fantastic to have an absolute out-and-out comedy in a season whereby you might think, ‘This Kenneth Branagh, he’s very famous for his Shakespeares.’ There are usually those two masks: comedy and tragedy, but the tragedy one gets all the high marks, but to pull off a comedy is tough business.”

Branagh shoots a question back to Brydon. “What is your experience of being in something that is playful but which requires a big [theatrical] mechanism too?” Not one for 'Question Time'. Brydon responds: “I have come to like it. In this we can’t [ad lib all the time] as there are so many bits that if you say a line differently, it’s like music, you can go off a note. It’s very hard to talk about without being pretentious. It’s like a watch!”

“Like a piece of Bach!” says Foley, laughing. Brydon shoots back: “But what tree?”

I ask about the clothes-swapping scenes (Brydon and Branagh end the play drenched in sweat wearing each other’s clothes). Are you the same size? Brydon puffs out his chest. “I am a little taller than Ken but not a lot.” Branagh: “We are the same shape. Well-built specimens. It was the first casting requirement.” Brydon: [in a West Coast accent] “I work out, sure I do!”

Foley interjects, still pondering on a previous conversation: “Eric Morecambe said, when someone complimented him on his ad libs, ‘I get very good at them the more I rehearse them,’ so it is that curious thing of being ferociously rehearsed and then it just looks like utter chaos.”

Both actors still get pre-show fright, but they have learnt how to banish it. Branagh says he gets fascinated with sports people, “those that go the extra mile while having total control. Great golfers who don’t bottle it under pressure.” Brydon explains: “If I sense a lack of confidence coming I tell it to f*** off. It’s there with all of us, this doubt, this feeling of ‘are you good enough?’ You have to have such confidence.”

Branagh continues: “You can say to Mr Gobby — Mark Rylance calls him [stage fright] Mr Gobby — ‘Stay in the dressing room.’ But as the Italians say, you wanted a bicycle, now peddle!”

“I’ve never heard that, that is great,” says Brydon.

I ask the three men what they are up to next. Brydon says he has another season of the comedy series 'Would I Lie to You?' and 'The Trip' to film with his partner in crime Steve Coogan, this time in Spain. Foley is finishing 'Mindhorn', a film written by 'The Mighty Boosh' star Julian Barratt and Simon Farnaby, in which he directed Branagh in the summer. “A very funny film,” says Branagh.

Brydon interrupts and turns to Branagh: “The elephant in the room, I suppose, is that you have very little coming up. Is it a quiet spell? Something will come, you don’t know who is coming to these plays. Someone will see you! They are doing a lot of good stuff in Holby [City] at the moment.”

Branagh plays it straight. “There might be something. We had lots of plays that we can’t fit into the season — an actor fell out, a play fell out, everyday stuff for this kind of thing — so we have other projects.”

I have read he is to star in a film about Dunkirk, written and directed by Christopher Nolan, who made the Batman films. “I believe I am not allowed to talk about it,” says Branagh.

“It’s out there, Ken,” says Brydon, helpfully.

“If I am in that I am very pleased but I am not allowed to talk about it,” Branagh repeats.

Brydon looks deep in thought. “So you are Batman? I was originally approached but it wasn’t big enough. Robin then?”

Branagh shoots back: “Rob was going to do it but then I believe you turned it down?”

“Robin Reliant, perhaps,” says Brydon.

Branagh changes the subject. “We hope to do a film of 'Murder on the Orient Express',” he says. Brydon, now in the sort of silly mood not seen since double maths on a Friday afternoon, auditions for a part.

“Can I offer you some breakfast, sir, in this carriage?” he says in an Italian accent.

“That is very good, actually,” says Branagh.

Is this a new double-act? How does Branagh compare to Steve Coogan? Brydon gives a mock-withering look. “Let’s not go there.”

“We like each other,” says Branagh.

The Painkiller is at the Garrick Theatre, London WC2, to April 30. Box office: 0844 4829673