In Ross Katz’s agreeable comedy-drama, written by Liz Flahive and Jeff Cox, Nick Kroll stars as a maturity-challenged entrepreneur who slinks away from career catastrophe back to his suburban family homestead, becoming the “manny” to a nephew he barely even knows. The boy’s parents, played with relaxed excellence by Rose Byrne and Bobby Cannavale, have a few little life crises of their own going on. It’s somehow both a help and a hindrance that everyone involved seems to understand this movie’s main hook — the prospect of man-child redemption — isn’t very sharp. Kroll’s intuition about how not to force things can be disarming, and sometimes even a little disorienting. Some scenes play like shrugs, which on the one hand levels their energy off, but on the other seems like the most honest gesture these occasions could evoke. Not surprisingly for a Duplass-brothers production, Adult Beginners gets its freshest juice out of low-boiled intra-sibling strife. Never mind the overstated and basically implausible swim-class crisis from which the movie’s portentous title derives; at heart it’s a simple chronicle of ordinary family dynamics. Bobby Moynihan steals one scene in a small but exaggerated supporting role, but that’s really because Kroll so generously gives it up.