I wonder what you would call fibbing as it relates to the feud between Dwayne Johnson and Vin Diesel.

A bald-faced lie?

I don’t know, but I’m pretty sure Stephen Merchant is skirting the truth when I ask him about the Diesel joke that turns up in his new movie Fighting with My Family, co-starring Johnson, who reportedly can’t stand his Fast and Furious franchise co-star.

Merchant wrote and directed Fighting, so he either authored the little Diesel zinger that Johnson (playing himself) drops during the movie, or he sat chuckling in his director’s chair as Johnson improvised it.

Merchant pleads amnesia.

“I can’t remember,” Merchant said, cheerfully and almost credibly, during a recent stop in Philadelphia to promote the movie. “It’s a collaborative medium, you know. And I don’t want to take credit if it wasn’t me.”

Merchant says he’s not even sure the feud between Johnson and Diesel is real, and if it is, he’s not sure it predates the completion of the Fighting with My Family script.

Sure. It’s all one big coincidence.

Anyway, what’s unarguably real is what you see in Fighting with My Family, drawn from a 2012 BBC documentary (The Wrestlers: Fighting with My Family) about Saraya Knight, a British woman (played by Florence Pugh) from Norwich who grew up in a family of low-rung professional wrestlers, learned the trade from mum (Game of Thrones’ Lena Headey) and dad (Shaun of the Dead’'s Nick Frost), and eventually auditioned for a WWE slot with her brother Zak (Jack Lowden).

Saraya, better known by her stage name Paige, gets the coveted gig, her brother does not, and the film goes on to tell the parallel story of Paige’s grueling WWE tryout ordeal (under the guidance of Vince Vaughn), and of the rift that develops between a brother and sister once so close.

Merchant is known for his work in comedy — he originated The Office in the UK with pal Ricky Gervais — but he pushes the dramatic elements in Fighting with My Family.

“I hate to use the dreaded word dramedy, but it fits. I don’t think I’d classify it as one or the other. Certainly, I loved the opportunity to work on dramatic scenes. As a fan, those are my favorite movies. I love The Apartment, Billy Wilder. It’s funny tonally but then halfway through, Shirley MacLaine tries to hill herself, so it’s a pretty dark film in many ways. For me, even going back to The Office, there were always moments of real sadness and real emotion.”

Merchant says he’s been influenced significantly by GoodFellas and The Sopranos. You can find funny moments, he notes, but all the humor is grounded in realism, which was a priority in Fighting, a movie that expands on the gritty tone of the documentary, which Johnson saw and loved and immediately envisioned as a feature film.

It would have been an easy story to Hollywoodize, or at least Americanize, but Johnson wanted to keep it close to its cultural roots. He thought of Merchant — the two have been friends since costarring in Tooth Fairy in 2010.

“To his credit, he wanted someone British to sort of give it that authenticity,” Merchant said.

Merchant acknowledges that he was not at the time an authentic wrestling fan. He was charmed by the documentary, but it wasn’t until he attended a live match with Johnson that he “got it.”

“Someone described [wrestling] as soap opera in spandex, and once I heard that, it made more sense. And I went to WrestleMania with Dwayne, and once you see it live, you see how complicit the audience is in the stories being told, the way everybody understands the levels of reality, creating a knowingness that is unique to wrestling. It’s a very special kind of energy — everyone is part of the willing suspension of disbelief, and of course there is great showmanship. I was very much won over by it."

His challenge was to create that energy on screen, but also to articulate the human story underneath — showing how a deep bond between brother and sister is frayed when one climbs the ladder to stardom and one does not.

As Paige goes on to the WWE circuit, Zak stays behind, and Merchant’s script devotes time to his less glamorous but worthwhile mission of using wrestling to help at-risk youth and the disabled.

“You know, it’s always a bit rich when someone like me, who’s in the public eye, starts to pontificate about fame and how it’s not important, but I do believe that. And I put that belief into Zak. I went to visit that wrestling school, and I saw those people, and they worship him. The stories he told about what he does were very moving to me. There’s no manual for teaching a blind kid to wrestle, but he did it.

“And I think it’s an important message. We live in this time when more and more, being a celebrity is seen as the most important thing you can be, the thing that validates you. But this is a guy who was doing really important work, making a real difference in people’s lives. And so as much as this is a movie about Paige, it’s also a movie about him, because his story is valuable too.”