David Harbour, who plays Red Guardian in Avengers: Doomsday, appears to have let slip a major spoiler about Robert Downey Jr's role in the upcoming Marvel film. Asked by Variety to deliver a line from the movie "out of context", Harbour adopted his character's Russian accent and said "You… are Tony Stark" – seemingly contradicting Marvel's marketing, which has insisted Iron Man is dead and that Downey Jr is instead playing the villain Victor von Doom. The remark has reignited fan speculation about what Marvel is really planning for the character, at a time when the studio badly needs a buzz-generating twist to boost interest in the film.

The most likely explanation is that Harbour was trolling fans, since the newly released Doomsday trailer, shown at Disney's D23 event in Anaheim, doubles down on Downey Jr playing Doom, complete with the character's accented voice and backstory. However, eagle-eyed viewers have spotted inconsistencies, including a masked figure with blue eyes rather than Downey Jr's brown ones, and a costume shown at D23 that differs subtly from the one displayed at San Diego Comic-Con in July. This has fuelled theories that there may be two versions of Doom in the film, with online speculation naming blue-eyed actor Cillian Murphy, long fancast as Von Doom, as a possible second performer behind the mask.