So many elements in film-making have become so dependably fine—cinematography, editing, production design—that by now only the exceptions are surprising. Screenwriting is a great deal more variable: the good work of the designers and others is often wasted on trash. Acting, however, is less variable, because most film scripts don't demand much more than verisimilitude from the cast, and many film actors, especially those with salable personalities, are skilled in what might be called behaving—without much distinction between what is on camera and what is off. But occasionally a role comes along that demands of the actor a process much more exigent than mere credibility, that requires investigation of self, creative process, risk.

One of the most pleasant experiences for a viewer is to see an actor who has sailed along expertly on personality shunt it aside in order to embody someone else. It isn't necessarily a matter of costume or accent or any of the easier changes. In one of the least appreciated gems of recent American acting, Jack Nicholson became Jimmy Hoffa. Nicholson did use an accent, a prosthesis, and a physical pattern unlike anything he had previously done, but he went through a much more subtle metamorphosis. Every moment that Hoffa was on screen, he seemed to be carrying his whole past life along with him, as basis and cause. Earlier in his career, when Nicholson played an ex-astronaut in Terms of Endearment, he had carefully coaxed his usual easygoing attractive self to grow over-ripe and pulpy, like fruit left out of the refrigerator too long. Here there was no accent, no false teeth—just the effluvium of disappointment and self-disgust that floated toward us on nylon charm. (And his achievement was inevitably even less noted than his more evident one in Hoffa.)

This matter of metamorphosis—becoming someone else—is far from the whole of acting, but it comes to the fore just now because of two current performances. The first is Ben Kingsley's in House of Sand and Fog. Kingsley's capacity for self-transformation has been marvelous for years: most of us became aware of someone else in his body before we knew much of the original Kingsley. His Gandhi, whatever one thought of the film as a whole, seemed to be a purification, a disregard of acting, even while it was of course a manifest of talent. Then came, among others, the smart London publisher in Betrayal, the Turkish spy in Pascali's Island, the quietly heroic Jewish accountant in Schindler's List—performances that enrich the lives of viewers. (On Broadway in 1983 he tried with his whole soul to play the great English actor Edmund Kean, but he couldn't quite achieve the genius of the man who, said Coleridge, revealed Shakespeare "by flashes of lightning.")

Now Kingsley appears in House of Sand and Fog, an odd piece by Vadim Perelman derived from a novel by Andre Dubus III. Quite clearly Kingsley took the role because of its challenges—it couldn't have been because of the cogency of the script. He plays an Iranian, formerly a colonel in the Shah's air force, now an American citizen living in Los Angeles, who works two jobs to support his family. He devises a scheme to make money by buying at auction a house that the state has seized for delinquent taxes, refurbishing it, and selling it at a large profit. The plot that follows, including the wretched young woman who lost the house, is of interest only insofar as Kingsley supports the structure with a powerful man. The colonel's accent is the easiest part of it, but the voice, the timbre, the touch of authority now kept in check, the ethos of a very different culture, the quiet superiority to the society to which he is nonetheless grateful for security, and a range of emotional truth from amusement to immense grief—all these are matters that are present without being displayed as actor's achievements. They are simply the man before us. To do that kind of acting requires long experience plus the ability to subsume one's talent.

Charlize Theron in Monster is a quite different subject. She is an especially attractive young woman who, up to now, has done quite adequately everything required of her. (Including a role in a Woody Allen picture.) Most of us, if asked to choose an actress to play a blowsy, overweight roadside prostitute who becomes a serial killer, would not have put Theron high on the list. But a first-time director named Patty Jenkins, who had made only a couple of shorts, chose Theron.

Jenkins has been justly praised for the screenplay that she wrote, based on the life of a woman named Aileen Wuornos, and for her close, immediate directing, but she has been insufficiently admired for her perception about Theron. The director must certainly have worked with Theron on her performance. The lovely Theron put on at least twenty-five pounds and was appropriately stained and bedraggled. This could be done to anyone. With Jenkins's help, Theron makes every word from Wuornos's mouth—usually foul—simmer up from the ghastly background and the vicious struggle of this life.

Wuornos is a woman who lives by instant gratifications, blasting changes of mood, excavating hollowness of spirit, and moral numbness. She murders some of her customers, first in self-defense, afterward in mere irritation, never with anything like regret. So intense is Theron's acting that she changes the film itself. From the start her performance is pungent, but we feel at first—about the picture as a whole—a sense of reportage. (In fact two documentaries have been made about Wuornos, neither of which I have seen.) But as Monster proceeds, the documentary quality fades. Wuornos ceases to be a social type about whom something should be done. She becomes a wretched individual facing one day after another with an anger that cloaks fright. Theron's performance changes the film's texture to the drama of an individual.

A plot winds around Wuornos's life, mostly concerned with a young lesbian from an orderly home who is played with tenderness by Christina Ricci, but it is Theron who transmutes and sustains this journey through the lower depths. Her work here piques curiosity about her future. She has already made another feature and a television film that apparently return her more or less to the Theron we knew. Fine; but Monster makes us hope that there are other anomalies in her future.

Wuornos was eventually arrested in Florida, tried, and sentenced to death. Toward the end of her lengthy stay on Death Row, Jenkins corresponded with her. After the execution in 2002, Jenkins gained access to her letters and occasionally a voice-over quotes from them. Essentially, those quotes sound like a woman striving for bravado on the road to inescapable hell.

Stanley Kauffmann is the film critic for The New Republic.