The controversial social psychologist Stanley Milgram gets a biopic as polymorphous as one of his own research studies in “Experimenter,” a highly formal, always fascinating movie from writer-director Michael Almereyda, who here delivers his most fully realized effort in the 15 years since his modern-dress “Hamlet” starring Ethan Hawke. Almereyda conceives of Milgram’s life and work as a kind of constantly evolving theater piece and runs with the idea, resulting in a decidedly Brechtian bit of filmmaking that routinely breaks the fourth wall and employs other bits of theatrical artifice to tell its tale. Such old-school indie-art-movie quirks won’t be to everyone’s liking, but for those who imbibe, “Experimenter” offers a heady brew of theories about the essence of human nature, and a Peter Sarsgaardperformance that catches Milgram in all his seductive, megalomaniacal brilliance.