"Entertainment Tonight" The Entertainment on the Red Carpet for the premiere of the New Marvel movie co-star had appeared in one of his most beloved films, "13 Going on 30," which debuted in 2004.
"She was a mean girl," said Larson in character from the movie. "Oh my God, I did not know that."
Larson reprises her role as a stand-alone movie starring Carol Danvers / Captain Marvel in "Avengers: Endgame" and Ruffalo is back as Bruce Banner / Hulk.  The now-29-year-old actress was pretty young when she was in the movie and Ruffalo, 51
"We did not work together," he said.
"13 Going on 30" starred Jennifer Garner as a young girl who wakes up as an adult and reconnects with her now so adult childhood friend, played by Ruffalo.
"Avengers: Endgame" hits theaters Friday.