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Matt Damon was initially set to direct and star in the film. Back in 2011, Damon, concerned about Kenneth Lonergan's 'horrible limbo', wanted to do something nice for his friend. So he sat in Lonergan's Manhattan apartment and pitched him an idea for a script about a New England handyman who ends up with custody of his dead brother's teen son - a story Damon thought would be right up Lonergan's dark alley. The idea for Manchester by the Sea actually was hatched by John Krasinski. Krasinski shared his nascent pitch with Damon over dinner in 2011 during the filming of The Adjustment Bureau (2011) (in which Damon co-starred with Krasinski's wife, Emily Blunt). They actually grew up a few bus stops from each other in Massachusetts - Damon in Cambridge, Krasinski in Newton. Damon liked Krasinski's idea so much, he considered not only starring in the film but also making it his directorial debut. They wasted no time heading to New York to pitch Lonergan. Embroiled in his Margaret (2011) troubles (the film was being held up from release by a lawsuit filed by producer Gary Gilbert, who was demanding a shortened re-edit that Lonergan was refusing to make) and committed to other stage projects, it took Lonergan a couple of years to get a first draft written. By then, Damon's schedule had filled up and he'd had second thoughts about directing the project, although he still wanted to star in it. Lonergan, meanwhile, finally had seen movement on Margaret (the film had a limited release in 2011; the court case was settled in 2014) and was starting to think about stepping behind a camera again. "He came back with this 150-page draft. It was too long, but it had everything that was in the movie." Damon urged Lonergan to make Manchester by the Sea (2016) his next directing job. Even with Damon still attached to star and produce, financing was hard to come by. And then, another blow: Damon had to drop out as the lead because of a slew of other commitments that would have delayed production for years. He recruited childhood friend Casey Affleck to take his place.
Kenneth Lonergan was going to cut the confession scene, but after Casey Affleck explained the importance of why his character chose to not drive the car and that he put a log on the fire because didn't want Randy's sinuses to dry out, Lonergan kept it in the film.
The town was called Manchester until 1989, when resident Edward Corley led a highly controversial campaign to formally change its name to Manchester-by-the-Sea. The action was passed by the state legislature that year.
Surprising everyone on set, Casey Affleck unexpectedly cried during the morgue scene. The brutal, but yet beautiful scene had been written without it, but Kenneth Lonergan decided to keep the scene.
While portrayed as working class and blue collar, Manchester-by-the-Sea is actually a very upscale and affluent town.
Matt Damon and Casey Affleck continuously discussed how deeply connected Lee remains to Joe, even though he isolates himself from the rest of the world. His brother's last act is an attempt to have Lee reengage with the world.