“Anyone who has married anyone knows you are going into a rival tribe,” laughed Colin Firth, explaining the theme of his new movie “Easy Virtue.” The handsome father-in-law in the venomous Whittaker clan in directorStephan Elliott’s remake of the 1924 Noel Coward play is married to a bitchy and funny Kristen Scott Thomaswho was directed to play her role more Disney witch. Firth has a thing for his son’s American bride, Larita, played ably, her performance evoking Kathryn Hepburn, by Jessica Biel. As the prince of period movies, the quintessential Darcy—see “Pride and Prejudice,” (and he’s pretty great in the Bridget Jones franchise), Firth has occupied many a fan’s dreams, but here he is “a fellow who has gone feral,” he says, he is out of step, like a Hemingway character with post- World War I scars and it isn’t until his son (Ben Barnes) comes to the country estate with Larita that he feels a zest for life beautifully illustrated in a sexy tango the two dance amidst the uptight others, jaws amply dropped. Together, Firth and Biel represent the future, characters liberated from ideals that no longer work. In this time of mortgage crises, the Whittaker’s attempts to hold onto their estate will be resonant. As to his film choices—and you have to think of his role in last summer’s “Mamma Mia”—he regrets only one he turned down: “Priscilla, Queen of the Desert.”