As "The Avengers" clip released last month featuring an epic battle between Iron Man and Thor proved, super heroes can have super tempers. And now the movie's writer/director Joss Whedon has filmed a promotional video for the film explaining the motivation for the super-showdown between the god of thunder and the armored crimefighter.

While staying mum on the source of the rivarly, Whedon reveals that it was crucial for the feud to spring from a genuine, personal rift between the two.

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"For me the most important thing was the fight was earned, that both of them had an agenda that conflicted with the others, that it wasn't a spell or case of mistaken identity, or something where you're just waiting for them to figure it out," Whedon explains. "They both really think they need to win, and neither of them is totally wrong, and that's what makes it more than a fight; it makes it a conflict."

Whedon gives particular credit to Chris Hemsworth, who plays Thor in the film, for giving the scene an authentic viciousness.

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"Chris' physicality, his intensity is extraordinary," Whedon says. "Thor is losing it; he's really losing it. By the end of the fight, he's really seeing red, and that explains the escalation of the thing. And it helped me work with Chris to make the fight go somewhere, and not just be a series of blows."

Plus, Hemsworth knocks noggins with his co-star Robert Downey Jr., despite the latter's head being sheathed in super-strong iron.

"To be head-butting a guy with a helmet on? That's badsass," Whedon notes.

"The Avengers" hits theaters like an oversize hammer on May 4. In the meantime, watch Whedon lay out the fight scene in the video.