The film, directed by Kenneth Lonergan, has received rave reviews already following its premiere at the Sundance Film Festival on Saturday (23Jan16).

While the movie stars Casey Affleck, Gretchen Mol and Michelle Williams, Matt Damon was originally slated to take on the lead role of Lee Chandler - whose life is turned upside down when he is tasked with looking after his nephew following the death of his father.

Matt opened up about his involvement in the project in a question and answer session after the screening, and admitted he just wanted to have something to do with the film in some capacity.

"The idea was for me to direct this movie, and I think John (Krasinski) was going to be in it, and then I think I was going to be in it. Once I read a very, very rough draft — it was about 4,000 pages — I just begged Kenny, I said you have to direct it. He eventually saw the wisdom. Then I was going to be in it, Kenny (Lonergan) was going to direct me in it, and in a bizarre and typical kind of fit of generosity, I gave it to Casey."

It was after reading the script for the film that Matt knew he was onto something special. And the actor added he can gauge the potential success of a movie by the reaction he has to its story.

"It’s one of the best roles I’ve ever seen, he insisted. "Basically, what happened - I had a full slate. I wouldn’t be able to do this movie until next year. They could go last year with Casey. I didn’t want to get in the way of a great movie getting made. I already said to Kenny, I wouldn’t give this role up to anybody but Casey Affleck, and Casey instantly took the role.

"I never made it through this script without crying, and I never made it through any iteration of any cut of this movie without crying. It’s absolutely devastating. It’s one of the most beautiful things I’ve ever come across. I wish I was in it, but I’m happy to just be attached to it."