In Kenneth Lonergan’s movies, the past gnaws at people until it threatens to gnaw them away entirely. This long, heartbreaking drama by the writer-director of Margaret (2011) and You Can Count on Me (2000) centers on a seething, tight-lipped young janitor (Casey Affleck) who arrives in the title Massachusetts seaport to bury his older brother and discovers that he’s been named legal guardian of his 16-year-old nephew (Lucas Hedges). The protagonist recoils from the idea of fatherhood, and the stress of this unwanted responsibility makes him the wrong guy to jostle at a bar. A playwright by training, Lonergan writes leather-tough dialogue that serves mainly to protect the characters from their own raw emotion; despite all the hard talk, this is a movie of deep sorrow, compassion, and forgiveness. With Michelle Williams, Kyle Chandler, Gretchen Mol, and Matthew Broderick.