Steppenwolf hand Terry Kinney gets behind the camera for this actor-y take on mortality and the slow waning of one’s faculties with “Diminished Capacity,” essentially a two-hander sitcom, adapted by Sherwood Kiraly from his own novel, between Matthew Broderick as an unlikely Chicago newspaper columnist and Alan Alda as his uncle in rural Missouri, whom he visits after suffering a concussion. Uncle needs to go into assisted living; script needs to go for rewrite. Alda’s a marvel, and I hope he lives to be in half-a-dozen if not more Martin Scorsese movies, but despite his best instincts here, crap plotting about Cubs baseball cards and other such unconvincing nonsense make this lethargic movie ultimately less than trivial. With Virginia Madsen, Bobby Cannevale, Dylan Baker. 92m. (Ray Pride)