The Hulk actor was speaking to ET when he was handed a still from the 2004 romantic comedy 13 Going on 30, in which Ruffalo starred opposite Jennifer Garner.
The image showed the "Six Chicks", members of the most popular clique at character Jenna Rink's school. Brie Larson was credited in the film as "Six Chick" rather than with a full character name, and had a line where she tells clique leader Tom-Tom that her yearbook photo is "great".
Ruffalo laughed as he was handed the photo, exclaiming "she was a mean girl!" in the film.
"Oh my god, that's amazing. I had no idea," he said. "I didn't know that."
Larson stars as Captain Marvel in Avengers, while Ruffalo reprises his role as The Hulk in what is being dubbed as one of the most emotional blockbusters in years.
“I have a lot of my friends and family here tonight, and a lot of them are right up here,” he told Variety. “We have this bond that transcends what we do for a living… I cried like six times [tonight].”
Marvel has continually teased Endgame as not only a sequel to 2018’s Infinity War, but as an “epic conclusion” to the 22 films to have been released before it.
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