Author Malcolm Gladwell has segued into Hollywood as the writer/producer of “Entourage” creators Mark Wahlberg’s and Stephen Levinson’s latest HBO show, “The Missionary.” But he’ll get a chance to show off his screenwriting chops before the show airs next year by appearing with film and TV vets — including Liev Schreiber, “Blue Bloods” director John Polson and writer Charles Randolph — at a symposium about “storytelling on screen” called Roughcut at the Elinor Bunin Munroe Film Center on Dec. 7. The event is part of Polson’s 20-year-old traveling short-film festival, Tropfest. Gladwell’s new HBO show stars Benjamin Walker as a missionary caught up in the Cold War.