In the new FX drama “The Americans,” premiering Wednesday at 10 p.m., Keri Russell (“Felicity”) and Matthew Rhys (“Brothers & Sisters”) play Philip and Elizabeth Jennings, married sleeper KGB agents in Reagan-era Washington. While Elizabeth is dedicated to Mother Russia, Philip seems to have doubts. At a recent Television Critics Association press tour in Pasadena, Calif., Rhys jovially fielded questions about being a Welshman playing a Russian playing an American.