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.
Kenneth Lonergan includes one of his past parenting experiences with his own daughter in the script. He lost his temper with his daughter who had lost something and a passerby across the street said, "great parenting" to him and Lonergan yelled back. In this film, Lonergan has a cameo as the man who heckles Lee for his bad parenting.
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.
While portrayed as working class and blue collar, Manchester-by-the-Sea is actually a very upscale and affluent town.