Though depression lurks around the edges of “The Instigators,” Doug Liman’s heist movie is a loosely amiable return to South Boston for Matt Damon and Casey Affleck, who also co-wrote the film. First, “The Instigators” works in a large percentage of today’s top character actors — among them Michael Stuhlbarg, Alfred Molina, Ving Rhames, Toby Jones and Paul Walter Hauser …