**1/2 My Week with Marilyn Michelle Williams is convincing and moving as Marilyn Monroe, circa 1957. The film itself is a stodgy period piece and self-defensive bit of grave-robbing on the part of memoirist Colin Clark (played as a young movie-set gofer by Eddie Redmayne), but Williams gets both Monroe’s insecurities and mystery. With Kenneth Branagh as Laurence Olivier. (99 min., R) (Ty Burr)