A host of thesps and execs did their patriotic duty last week, turning out for the preem of the Warner Bros. comedy “My Fellow Americans.”The Jack Lemmon-James Garner starrer screened Wednesday at Westwood’s Mann Bruin, followed by a party at Chasen’s. Munching on all-American fare from the ever-present Along Came Mary were the pic’s stars as well as cast members Dan Aykroyd, with his wife, Donna Dixon; Lauren Bacall; Sela Ward; and Marg Helgenberger. Helmer Peter Segal and producer Jon Peters also was on hand with exec producers Craig Zadan, Neil Meron and Tracy Barone, there with her new husband, Paul Michael Glaser. WB sent a flotilla of execs, including co-chairmen, co-CEOs Bob Daly and Terry Semel; and co-chiefs of worldwide production Lorenzo di Bonaventura and Bill Gerber. Spotted in the crowd were James Coburn, Roddy McDowall, Vanna White, Cathy Moriarty, David Spade, Courtney Thorne-Smith and Laura Leighton.