Los Angeles: Actor Mark Ruffalo will star in and executive produce limited drama series “I Know This Much Is True”, which will see him play identical twins. “I Know This Much Is True” is based on author Wally Lamb’s similarly titled best-sellger. It will be a six-episode series for HBO, reports hollywoodreporter.com. The family saga follows the parallel lives of identical twin brothers in what is described as an epic story of betrayal, sacrifice and forgiveness that is set against the backdrop of 20th century America. Ruffalo will play twin brothers Dominick and Thomas Birdsey. Derek Cianfrance will pen the script, direct and executive produce the project alongside Ruffalo, FilmNation Entertainment’s Ben Browning and Glen Basner, Lamb, Gregg Fienberg and Anya Epstein. Lynette Howell Taylor and Jamie Patricof will co-exec produce.