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.
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.
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.
While portrayed as working class and blue collar, Manchester-by-the-Sea is actually a very upscale and affluent town.
According to an interview with Kenneth Lonergan on DP/30, the idea for the film didn't originate with him - the main core of a character going back home to take care of a family member after a death was pitched to Lonergan by Matt Damon and John Krasinski as a script that Lonergan would write and for Damon to direct; but, due to scheduling conflicts with The Martian (2015), Damon couldn't direct the film or star in it. (He was considered for the role of Lee that Casey Affleck was cast in.) Lonergan was then given free rein as a writer-director for the project, with Damon as producer.