"This screenplay was what brought us to The Descendants-it became our calling card - and we always wanted to get back to it. It was a passion project for the most part. Over the years we've been adding things and fleshing things out but it's still pretty similar."
The film follows Duncan (Liam James), an awkward 14-year-old who doesn't have many friends until he meets Owen (Sam Rockwell), a layabout with a heart of gold who works at a run-down water park.
It provides relief from his fractured family life - Duncan's single mum Pam (Collette) is trying to make it work with her creep of a boyfriend (Carell).
"'From now on I'm only playing creeps," jokes Carell, who is surprisingly lacking in humour here.
Rash was grateful to Carell for coming on board.
"Steve helped us bring the character to life - and it was quite tricky," he admits.
"Trent is really designed as a man who in his life has hit a circle that he can't break out of. Obviously there are characters that are always going to change by the end - that's the point of making great stories. But for Trent ... his own worst enemy is himself and every time he thinks and sees something he has that's so wonderful in front of him, he ruins it."
The film was shot in Marshfield, Massachusetts, to accommodate Carell's family summer holiday plans. The town's Water Wizz park plays itself.
The setting was a nostalgic one for Faxon: "Summers at water parks seem like a great place to give Duncan an escape because for us, growing up it was one of those things where your parents would drop you off, put a 20 in your hand and say, 'See you later'.
"You just would have an amazing time at those places. No matter what it looked like to anyone else, to you it was wonderful."
Some episodes from Rash's own youth, who is in the cast of the US sitcom Community, inspired scenes. Like the one in which Trent cruelly asks Duncan how he rates himself on a scale from 1 to 10.
"That was inspired by a real event my life," Rash recalls, "when I was asked what I thought I was on a scale of from 1 to 10 by my stepfather at the time during a car trip to Michigan.
"We knew that was a great launch for understanding Duncan's journey."
- additional reporting: AP