It’s become increasingly common for actors to bulk up and slim down for desirable roles; Robert De Niro has altered his appearance repeatedly in this way (just recall his beefy Al Capone of “The Untouchables”), and more recently Renee Zellweger and Tobey Maguire have made similar sacrifices on behalf of Bridget Jones and Peter Parker. But not since Vincent D’Onofrio gained so much weight to play Private Pyle in “Full Metal Jacket” that many viewers didn’t even recognize him as the same guy who was the Thor-like Dawson of “Adventures in Babysitting” (released the same year) has a performer transformed himself or herself so radically as Charlize Theron does in “Monster.” The stunning young blonde not only added considerable poundage but was supplied with blotchy skin and buck teeth by makeup wizard Toni G. to take on the role of Aileen Wuornos, the woman often termed the first female serial killer, who was executed in Florida in 2002. Comparisons with news footage and the pair of documentaries done on her case by Nick Broomfield, in fact, suggest that Theron’s version of Wuornos is actually weightier and more unattractive than the real thing. She appears a more volatile, rage-filled person, too–more continuously snarling and snappish.

But whatever exaggeration exists is relatively slight, since her actions show that the actual Wuornos was a highly charged woman capable of great violence. And under any circumstances Theron’s performance is a real tour de force, a star turn of unquestionable power and punch. But it’s not just a calculated impersonation–it carries true emotional resonance, proving beyond all doubt that she’s an actress, not just a beautiful celebrity made up to look dowdy and plain.

As a whole, however, Patty Jenkins’s film doesn’t match her contribution to it. One can understand the effort to humanize Wuornos by portraying her, at least in part, as the result of familial, socio-economic and anti-female forces at work in the larger world, but the script seems pretty much to accept the argument that her seven murders of men who had picked her up along the highway, mostly for sex, were merely the result of either brutality done to her by them or her fear (either of more abuse by them or of exposure)–a very doubtful proposition. It also ratchets up sympathy for her by showing another side: when she’s confronted by a different sort of john, a stuttering, sad fellow (Pruitt Taylor Vince) who’s more terrified than aggressive, she not only services him with a touch of kindness but lets him live. Though the episode might be useful to Jenkins’s softening of Wuornos’s character for dramatic reasons, one can only wonder how accurate it is. But there is at least some compensation in the portrayal of Wuornos’s final killing, of a good Samaritan who has no desire but to help her. Of course, it’s made clear that by this time she was such a physical wreck her judgment was severely impaired.

“Monster” also stumbles in its depiction of Selby Wall, the woman whom Wuornos loved so madly that she’d do anything for her–from trying to get a regular job (with dismal results–one of her interviewers is a smug, cruelly dismissive type) to killing her customers for cars and cash. Christina Ricci is a fine young actress, but she’s all wrong for Wall, who–as the news footage shows–was hardly the mousy, recessive type shown here (and who, as the record indicates, might have had a much more direct role in the murders than Jenkins chooses to indicate). Ricci works hard at the role, but she never takes hold, and since the picture is basically a duet, the imbalance hurts it deeply. Among the supporting players, you’ll certainly notice Bruce Dern, cadaverous and with a huge mane of white hair, as one of Aileen’s few friends; Vince, who puts his patented bulky but timid persona to good use as the terrified trick; and Scott Wilson (once a screen killer himself, in Richard Brooks’s memorable film of “In Cold Blood”) as Wuornos’s final victim. More caricature-like are Lee Tergesen, as the john whose viciousness leads to the initial killing, palpably in self-defense as shown here, and Annie Corley, as the family friend with whom Wall is living before taking up with Wuornos. She’s the very image of the bigoted, uncomprehending redneck spouse. Technically, apart from the superb makeup work, the film is just ordinary, though the visual drabness is doubtlessly a fitting reflection of the actual milieu.

There’s no doubt that “Monster” is a fine showcase for Theron’s powerhouse performance, but in comparison to the great true-crime pictures of the past, it comes up short.