The actor also shares what it's like playing a talking flower in his new movie, 'IF.'
Matt Damon and John Krasinski may be starring in IF together, but the two aren't imaginary friends.
Damon dished on what it was like to play a walking, talking flower named "Sunny" in his real pal's directorial debut during its grand premiere in New York City on Monday.
"He showed me the picture probably a year before I did the voice and he showed me Emily's picture," Damon told ET's Rachel Smith. "He had all the renderings really early but he had been talking about the movie for. or working on it, it's gotta be four years in the making. I mean, he put so much of his heart into it. It's so beautiful. It's so him. It was a pleasure to just be able to be a part of it."
The actor was asked which aspect of his youth he misses the most, outside of imaginary friends.
"Oh man, some of the people," he answered. "Some people move along, you know what I mean? In one way or another and so there's always a nostalgia when you look back."
Damon, 53, was joined by his wife, Luciana, 47, at the prestigious SVA Theater. The red carpet event also included Ryan Reynolds, Steve Carell, Maya Rudolph, Christopher Meloni, Awkwafina, Bradley Cooper, Amy Schumer, Cailey Fleming, Fiona Shaw, Alan Kim and Bobby Moynihan.
"There's a ton of us in there but everybody wanted to be a part of it for sure," Damon said.
The Martian star was at another star-studded affair just last week -- the 2024 Met Gala at the Metropolitan Museum of Art on May 6. He opened up about the highlights of the night.
"There were a lot," he revealed. "There were really wonderful people there. People I didn't know I got to meet -- Andrew Scott, an actor who played the same character I played 25 years ago, so that was really fun. And he was actually there with Jude Law who I did the movie with. It was very funny, we were all seated at the same table so that was definitely a highlight."
And while it wasn't Damon's first Met Gala, it was definitely a memorable one.
"It was fantastic. It was beautiful," Damon continued. "There was a great performance. It was great, it was, you know, a lot of people, no kind of pressure in there 'cause there's no cameras or anything, so everybody's pretty relaxed."
When it was brought up that Ed Sheeran was posing with nearly everyone throughout the night, Damon replied, "He's always a popular guy. I met him through Chris. He's great."
Damon's new movie, IF, is a whimsical journey into the world of imaginary friends (also known as IFs), seen through the eyes of a young girl with a unique gift.
IF hits theaters Friday, May 17.
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