Heath Ledger had an unusual ability to mix lightness and gravity, an emotional nimbleness he displayed most fully in Todd Haynes’s “I’m Not There.” Of the six avatars of Bob Dylan in that film, his, an actor named Robbie Clark, is the most remote from Mr. Dylan’s various personae and closest to the prosaic world of love, fame and ambition. Robbie starts out full of youthful energy, heedless and in love, and finds himself a decade later adrift and disappointed, robbed of the happiness that early success had seemed to promise.