Beauty would seem to be a virtual requirement for movie stardom, but for some actresses it can be a career bump they never get over. I've always thought that Geena Davis, as statuesque as a Valkyrie, with that impossibly ripe kisser, faded as a star because she looked so unconvincing trying to impersonate a lawyer, a waitress, or just about anyone else. Charlize Theron has faced a similar dilemma. She's a baby-doll Amazon, like Marilyn Monroe on stilts -- an actress who appears fundamentally incongruous whenever she tries to pass herself off as ordinary.
Theron may have realized the problem, because in Monster she undergoes one of the most startling transformations in the history of movies. As Aileen Wuornos, the Florida prostitute who murdered seven men in the late 1980s, becoming tagged in certain media quarters as America's ''first female serial killer'' (she was executed in 2002), Theron has changed her body, her face, her skin. She's puffy and sluggish, with thick, matted hair; the ruddy, mottled complexion of a sunburned boozer; and a mouth that drops open into a look of slack disengagement. Aileen isn't just down and out; she's a piece of wreckage, a straggler gone to seed. Theron's physical alteration, which makes Nicole Kidman's in ''The Hours'' look like something accomplished with a store-bought Halloween costume, is a landmark for the makeup artists who achieved it, yet the fascination of ''Monster'' is the way that Theron matches that look from the inside out. She becomes Aileen Wuornos in all her rattiness, a slattern who hates herself and hates the world. This isn't just a performance, it's an act of obsession -- an actress parading demons we might never have guessed she had -- and it lends ''Monster'' a heightened aura of excitement, even if it isn't, in the end, a very good movie.
As a teenager, Wuornos, emerging from a nightmare of abuse and neglect, fed her hunger for approval by sleeping with boys for money, and she grows up into a prostitute of slovenly disrepute -- a drifter, often homeless, who picks up men by thumbing her way along Florida's Interstate 75. She's also in deep conflict over her sexual desires, at least until she meets the pretty, gullible Selby (Christina Ricci) at a lesbian bar. The writer-director, Patty Jenkins, allows these two a flickering moment of happiness as they skate around a roller rink to Journey's ''Don't Stop Believin.''' But the relationship becomes captive to Aileen's new fixation: picking up johns and shooting them dead. Her first homicide is an act of self-defense, after she is raped in a roadside encounter of scary brutality. Once her resentment of men is tapped, it flows like poison lava, and she can't keep herself from replaying that vengeance, over and over again.
''Monster'' makes the half-baked suggestion that Aileen, caught in her vile roadside jungle of desperate male predators, is some sort of a victim. Since all we really see, however, is a vicious and paranoid psychopath, the film lacks just what it needs most: a compelling point of view. Theron imbues Aileen with an animal desire for love, yet as anyone who has seen Nick Broomfield's 1992 documentary ''Aileen Wuornos: The Selling of a Serial Killer'' can attest, the actress has remained starkly true to Aileen's squalid, blinkered selfishness. She plays an unredeemable woman with uncompromising reality. That's a powerful accomplishment, even if ''Monster'' never quite figures out what to do with it.