Sundance: Here's why Matt Damon doesn't star in 'Manchester By the Sea'

PARK CITY, Utah — In Hollywood, Matt Damon and Ben Affleck go together like (A-list) peanut butter and jelly.

They're Boston childhood BFFs, the wunderkinds who wrote and starred in 1997's Good Will Hunting (which won two Oscars) and have since produced and/or starred in more than a dozen projects together.

But at Sundance Film Festival this weekend, all eyes were on a new Damon-Affleck collaboration, this one starring Casey Affleck. Manchester By the Sea, written and directed by Kenneth Lonergan (You Can Count on Me), was Saturday afternoon's hot premiere as snow began falling.

The New England drama stars Casey as Lee, a janitor who returns home when his brother, Joe (Kyle Chandler), dies suddenly. But even though Lee is the best (and sole) option to raise his teenage nephew (Lucas Hedges), he resists becoming his legal guardian, a bewildering prospect until the introduction of Lee's ex-wife, Randi (Michelle Williams), and the heartbreaking reason why.

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It's a juicy, dramatic role that Matt Damon was supposed to play himself. But then he gave it to Affleck (and took a producer role). Here's why:

The script took years, "like seven ... maybe five," said Damon during the Q&A. "Originally, the idea was for me to direct this movie. I think John (Krasinski, who is at Sundance with The Hollars) was going to be in it, and then I was going to be in it.

"(But) once I'd read a very, very rough draft, it was about 4,000 pages, I just begged Kenny, I said, 'You have to direct it.' ... And in a really bizarre, atypical kind of fit of generosity, I gave it to Casey. Which I immediately regretted."

Cue the audience laughter.

"Basically, it's one of the best roles I've ever seen," said Damon. "I had a full slate and I wouldn't be able to do this movie until next year and they could go last year with Casey. I didn't want to get in the way of a great movie getting made. And I had already said to Kenny, 'I will not give this role up to anybody except for Casey Affleck.' And Casey instantly took the role. Because he saw what we all saw. I never made it through this script without crying ... I wish I was in it, but I'm happy to be attached to it."

So how did Manchester play?

Oscar-focused predictions rolled in for Affleck on Twitter. The Wrap says Manchester "could inject life into what has been a dreary sales market thus far" at the festival.

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Indiewire praises the "magnificently messy, painfully real" story, giving credit to Affleck, who "has a difficult job in this film" and "maximizes the micro-moments."

"The electric applause inside the Eccles (Theatre) after Manchester By the Sea premiered was reminiscent of when Boyhood debuted there two years ago," wrote Variety.

No pressure there.