I’m not a good person,” sad-sack small-town Gerry informs his new pal, slick and sophisticated Curtis. But he’s not exactly a bad person, either: he’s an addict, and his addiction is to gambling. The writer-director team of Anna Boden and Ryan Fleck (It’s Kind of a Funny Story), in their best work yet, craft a compelling character study of these two intriguingly flawed people as they take a road trip down the Mississippi, from Nebraska to New Orleans, where Gerry is absolutely convinced that the secret high-stakes poker game Curtis can get him into is his one hope of winning the money he needs to pay back all the many people he’s in the hole to (including, in a smoothly gripping one-scene appearance, Alfre Woodard [Annabelle] as his bookie). Ben Mendelsohn (Slow West) is absolutely extraordinary in how he navigates Gerry’s irresistible compulsions and superstitious tics, but the real surprise here is Ryan Reynolds (Self/less), who lets us see past Curtis’s outer gloss into his hidden rough patches of loneliness, though we are rather fascinatingly left to ponder what Curtis’s own motivations might be; he is a gambler, but he’s not an addict like Gerry. The mystery of Curtis creates a marvelous, edgy suspense as to how Gerry is ultimately going to fare in his company. Such satisfying ambiguity is an uncommon trick to pull off, but Boden and Fleck manage another one as well: as Gerry and Curtis wend through places such as St. Louis, Memphis, and Little Rock, Grind becomes an example of a real rarity these days, a true American original with a palpable sense of place that isn’t New York or Los Angeles. This is the sort of grownup, character-driven drama, richly observed and smartly ambivalent, that has sadly gotten all but pushed out of mainstream cinema in recent decades: movie like this one would have been a major event in the 1970s. Bravo to Boden and Fleck — Mendelsohn and Reynolds — and for keeping the flame burning.