Bea (“This Way Up,” “Living With Yourself”) plays Lynn, the short-lived wife of Jack (Domhnall Gleeson), whose 15-year on-again/off-again romance with Alice (Andrea Riseborough) provides the central conceit of “Alice & Jack,” a twisty dramedy premiering March 17 on Masterpiece on PBS.“I think I sort of knew what [series creator] Victor [Levin] was getting at and … one of my passions with my writing and my acting is what is sometimes seen as a ‘side character,'” Bea, 39, told The Post. “Maybe the less dynamic characters that don’t get written about … you imagine them having a whole movie or TV show of their own.”The series opens with Alice and Jack meeting at a bar and hitting it off.