Zack Snyder has debunked a fan theory about Batman v Superman: Dawn of Justice, which is that the caped crusader’s former sidekick Robin had morphed into supervillain the Joker.

The theory had been gathering steam since the release of the superhero blockbuster, as a way of explaining the new Batman’s out-of-character behaviour: Ben Affleck’s Dark Knight targets Superman on minimal evidence and kills bad guys on the streets of Gotham.

Batman v Superman stars Ben Affleck and Henry Cavill: ‘Batman’s like Hamlet’ Guardian

At one point in the film, Batman walks past a Robin costumethat has bullet holes, in his Batcave. Fans have suggested that photographs of a topless, tattooed Jared Leto in character as the Joker for the forthcoming film Suicide Squad show scars on his left and right shoulders in the same position as the holes in the Robin costume.

But Snyder told IGN he has a more orthodox explanation for the scene, one that recalls the 1988 graphic novel A Death in the Family. “In my mind, it was that Robin had died 10 years earlier, during some run in with a young Joker,” he said. “To me, it was a fun backstory … to play with. The whole idea was that there had been loss and sacrifice.”

Snyder added: “[Batman] doesn’t really have a life outside of the cave. I thought by including Robin – a dead Robin – it would help us understand he’s been on quite a little journey.”

Affleck’s agent, Patrick Whitesell, told the Hollywood Reporter this week that the actor and Oscar-winning film-maker had written his own script for a Batman movie set in Warner Bros’ planned “DC Comics superhero universe”. However, he also said his client was currently only contracted for two more appearances as Batman, in two forthcoming Justice League films which are due in 2017 and 2018.

Batman v Superman has been heavily criticised for its lack of humour, notably by Affleck’s friend and sometime director Kevin Smith.

In related news, Birth.Movies.Death reports that David Ayer’s Suicide Squad movie is having reshoots, in order to lighten the tone and add more comedy.

Suicide Squad, about a team of DC Comics supervillains who are allowed out of jail to complete an “impossible” mission, has been the subject of huge interest since a trailer featuring an irreverent tone and plenty of bad-guy wisecracking hit the web. However, the fan site reports that the promo featured most, if not all, of the movie’s jokes, and that new scenes are being shot to ensure it doesn’t misrepresent the final movie.

Starring Margot Robbie as Harley Quinn, Leto as the Joker, Will Smith as Deadshot, Joel Kinnaman as Rick Flag and Cara Delevingne as Enchantress, Suicide Squad is due in cinemas this August.