From writer/director Derek Cianfrance and based on the award-winning novel of the same name by Wally Lamb, the six-part HBO limited series I Know This Much Is True follows identical twin brothers Dominick and Thomas Birdsey (played in two stand-out performances by Mark Ruffalo) on their own very different life paths. It’s a family saga of sacrifice and forgiveness which begins in their present — the early 1990s — and digs into different stages in their lives as it highlights how mental illness affects everyone, including the person directly dealing with it. Rosie O’Donnell, who plays …