One of the many successes of the sensitive and astute film Capote is that it hints at the cause of her putative disappearance: Capote murdered her with his profound negligence of someone who devoted so much energy to him.
That isn't what the movie is about. Nor is it about what a nasty person Capote was (as a stupid woman walking behind me leaving the movie theater said to her husband/date). Philip Seymour Hoffman creates a remarkable and Oscar-worthy picture of the man who could write and live the idea -- to paraphrase Capote's inscription in In Cold Blood -- that sometimes more tears are shed for dreams fulfilled than for those not realized.
After watching Hoffman and Catherine Keener as Harper Lee, I can't help but think that Lee may still be shedding tears for Capote's dreams fulfilled . . . and for her own.