Matt Damon said he was "totally fine" with missing out on the lead role in Oscar hopeful Manchester By The Sea.
The Jason Bourne actor was expected to star in the film set in the Boston suburb but was forced to pull out due to his busy schedule.
Casey Affleck replaced Matt in the lead role and is earning rave reviews for his performance of a grieving brother who becomes the legal guardian to his teenage nephew.
Matt, who is a producer on the film, insisted he would only have passed on the role for Casey.
"I said I wouldn't have let it go to anybody but Casey," Matt said at the movie's Toronto premiere. "That was really the condition.
"We were actually in pre-production but our time got so squeezed, we said 'all right, let's punt'.
"When we looked, our next window - it was two years away. I said 'all right, if we can get it together with Casey, I'm totally fine with that'.
"I knew he would just crush the role. It's a beautiful role. The right actor always gets the part.
"Probably as a producer, the smartest thing I did was getting out of the way."
Critics have hailed Manchester By The Sea since its premiere at the Sundance Film Festival earlier this year.
The film tells the story of a reclusive handyman - played by Casey - who must face his painful past when he returns to his Massachusetts home town after the sudden death of his beloved older brother.
Matt, 45, admitted the film - which also stars Michelle Williams - was a "really challenging movie to get made".
"It's not like other movies," he said. "That's what makes it great but that's what made it really hard to get funded.
"The kind of margins we're talking about in the movie business, you've got to have a lot of guts to get in the game."
Matt, who won an Oscar with Casey's brother Ben for their Good Will Hunting screenplay, said he was "thrilled" to hear talk of potential awards for the film's lead actors.
"They really do deserve that kind of chatter," he said. "They are really special performances and it's a special movie."
Manchester By The Sea is due to be released in UK cinemas in January.