MAKING ONE THING PERFECTLY CLEAR: We remember standing on line outside the Thalia on the Upper West Side two decades ago to see Robert Altman’s “Secret Honor.” It had a one-person cast, Philip Baker Hall, who gave what Post critic Archer Winsten at the time called a “mesmerizing” performance as Richard M. Nixon. “The actor gets down on his knees,” wrote Winsten, “prays to his mother, barks in memory of youthful games, and unleashes some damning indictments of Amercan politics and politicians.” To mark the 20th anniversary of the film, it is unreeling Sunday at 4 p.m. at Symphony Space (Broadway and 95th Street), which now houses a sanitized version of the beloved old Thalia. Altman will be on hand to answer questions after the show. For more information and to order tickets in advance, visit http://www.symphonyspace.org.