Taylor and Clift were both ascendant actors when they were cast, but their trajectories couldn’t have been more different. She was an MGM child star, trained from an early age to perform on cue. He was a child of the New York theater, a searching, deeply intelligent talent who, along with Marlon Brando, helped revolutionize American acting by turning it inward. His example inspired Taylor to deepen her own game, but, as Charles Casillo’s touching “Elizabeth and Monty” underscores, theirs was a relationship of equals. Not a screen romance, exactly — he was gay, and she was ardently and serially heterosexual — but the longing was mutual, and that truth still shines out from those long-ago close-ups.