The actor learned on Monday night at the Avengers: Endgame premiere in Los Angeles that he and the Captain Marvel leading lady had both been in 13 Going on 30 together. While on the red carpet, ET's Kevin Frazier asked Ruffalo if he knew that Larson was in the rom com, to which he admitted that he had no clue.
"She was a mean girl," a shocked Ruffalo said with a laugh as he looked at a still from the movie. "Oh my god, that's amazing! Wow, I didn't know that. It looks like her a lot. It's her."
"How cool, I didn't know that," he said before asking, "Did she talk about that? What did she say? We didn't work together."
"That was a lovely little movie. I didn't get to work with her," he added.
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ET also spoke with Larson at the premiere about her time on the 13 Going on 30 set, where she revealed it was the set that made her want to direct in the future.
"I remember it was like the moment when I realized that I really wanted to direct," she shared. "I remember paying a lot of attention to where the camera was and the camera moves, thinking about how I would have cut it together and then seeing the movie and seeing how it differed from what I thought."
Earlier this month, Ruffalo also touched on how amazing it was working with the film's lead, Jennifer Garner.
"We were making this really sweet movie that had a great message for young girls," he gushed. "We were all sort of coming into our own in that moment. We really loved that story, and Jen was just transformed into a 13-year-old girl. It was really amazing."