Mark Ruffalo Just Learned That Brie Larson Was Also In '13 Going On 30' Before Joining The Avengers — VIDEO

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Some people might see the upcoming Avengers: Endgame as the 13 Going On 30 reunion fans didn't know they needed, but Mark Ruffalo is not one of them. Just one day before the 15th anniversary of the romantic comedy, on Monday, April 22, Ruffalo learned that Brie Larson also starred in 13 Going On 30, a fact he never knew before Entertainment Tonight broke the news to him on the Avengers red carpet.

Ruffalo — who played Matty, the adorable best friend of Jennifer Garner's Jenna Rink — spoke with the outlet at the World Premiere of Endgame in Los Angeles, and actually revealed that Larson's current superhero character, Captain Marvel, is one of his children's favorite Avengers. But despite the many connections between Ruffalo and Larson, the former had no idea that they had appeared in the same movie long before Endgame. "She was a mean girl! Oh my God, that’s amazing. Wow, I didn’t know that," Ruffalo responded, referencing Larson's role as one of the Six Chicks, the popular clique of girls who pick on teenage Jenna (Christa B. Allen) and ruin her 13th birthday party.

Of course, Ruffalo had a good reason for being unaware of his past connection to the current Carol Danvers: "We didn’t work together [on that movie]," he told ET, due to the fact that Larson only appeared in the film's '80s scenes. "That was a lovely movie."

While he might not have remembered every single actor in the film, Ruffalo does clearly have fond memories of 13 Going On 30, as he celebrated its 15th anniversary on Tuesday, April 23, with a touching Twitter post. "Happy 15th anniversary to 13 Going On 30!" Ruffalo wrote, alongside an adorable gif of Matty smiling at Jenna in the film. "Still a sweet movie after all these years. Remake? 13 Going on 51." Ruffalo also gave a shout out to the film's late director, adding, "Rest In Peace, Gary Winick."

One person who did remember that Larson was in the movie, however, was Garner, who gave the Oscar winner a shout out in her touching tribute to the film and its iconic slumber party scene. "The movie benefitted from the performances of so many young actresses," Garner wrote on Instagram. "Open and twinkly-magical then— now, 15 years later, they are ruling the world. (Hi #SixChicks—@brielarson, @ashleybenson, @christabrittany—who also played a young me in Ghosts of Girlfriends Past—and my own sweet @revjuliaroth)."

Jennifer Garner/Instagram

Garner continued by highlighting the joy that filming the slumber party sequence with her young costars had on her both at the time, as well as 15 years later. "I loved my day with this slumber party crew: they danced with abandon and reminded me of the power of just saying yes—to being in a group, to vulnerability, to wearing a bra over your clothes," she wrote. "Imagine if every 13 year old girl had an opportunity to dance and feel beautiful like the girls in this film. I am proud of all of you."

Strangely enough, Ruffalo isn't the only Avenger that Larson has worked with in the past: during a recent appearance on The Ellen DeGeneres Show, Scarlett Johansson reminded Larson that they both worked on the film Don John together. "Clearly it made an impression on me, but somebody else didn't care!" Johansson said when Larson didn't immediately recall their scenes together. "We worked together!"

"Oh my god, it's true!" Larson responded. "I didn't even think about that because I just felt like I was like a glorified extra and you were Scarlett Johansson so I was just like, 'Wow!'" Despite the awkward moment on the show, the pair revealed that they are now very close friends, thanks to an 8-hour dinner on New Year's Eve where they bonded all evening.

Clearly, Larson was always destined to be a member of the Avengers team, as she kept crossing paths with her fellow super heroes long before she ever donned Captain Marvel's super suit.