I haven't read Nick Flynn's autobiographical book on which "Being Flynn" was based, so I'm addressing here only its effectiveness as a movie and not its adequacy or otherwise as a representation of the book. And on its own terms as a movie, it works well. In Paul Dano (as Nick Flynn) and Robert DeNiro (as his delusional father, Jonathan), it is very strongly cast, and in its attention to practical circumstances of being homeless or of living and working in a homeless shelter, it has a quality of slightly heightened documentary style that seems just right for a story about two men who consider themselves writers and whose writing seems to be concerned to give shape to their lives. When I say, "heightened," I mean that we get the sleeping on grates and in the garbage piles, but these are photographed quite artfully but nonetheless effectively, and our noses, so to speak, are not rubbed in the artfulness. Another virtue of the movie is its refusal of sentimentality. Jonathan has been absent from Paul's life for almost two decades, and he gets in touch only when he's in dire need -- he has become homeless. He's a self-centered, difficult, at times violent man, convinced of his genius as a writer, and not at all interested in his son's life. When Nick tells him that he too is a writer, Jonathan's response is couched in terms of the genius that he has transmitted and not on any achievement or potential in Nick himself. A lesser movie would set the relationship up for a climax in which mutual understanding and respect is articulated, but we get something well short of that uplift, though still moving enough. At the center of the movie is Nick's decision to get his aimless, stoner-ish life in order by working at a shelter (run by "the Captain," played by Wes Studi, whom I hadn't seen in a long time), only to have his father come to live in the shelter and behave often badly. Nick's response, even as he tries to do his job, is to revert to drug use and aimlessness, before pulling himself together, with the encouragement of Denise (Olivia Thirlby), a co-worker, and join an addiction support group. Just as the relationship with his father isn't led to a sentimental conclusion, so is the sentimental "salvation" by Denise's love avoided, and the crowded, semi-chaotic scenes in the shelter and on the street keep our feet on the ground.
Formally, the movie uses a double voice-over technique -- we get parts of the story told by Jonathan and parts by Nick. Nick's parts hearken back to memories of his life with his single mother (Julianne Moore, very effective); The delusional and self-centered Jonathan is focused in his present. Nick's fear, of course, is not that he'll lose his father but that he'll turn out to be just like him. Since the way their narratives are handled show that he ISN'T like him, we come to be concerned that nonetheless he might come to believe that he is, and that would be a kind of defeat. Whether or not that is avoided, you'll have to watch the movie to find out. Paul Dano is very good as a young man whose confidence comes and goes and whose belief in himself is uncertain. DeNiro, with a less complicated character, is nonetheless absolutely convincing as a delusional, self-involved man who has damaged lives without apparently being aware of it, and who reveals both how pathetic Johnny is and how unaware that that is how he appears. The characterizations are consistent up to the very last frames. Well worth a look.