In his feature-film debut, rapper and general American obsession Eminem plays Jimmy “Bunny Rabbit” Smith, a trailer-park-bred, aspiring emcee who has “one shot, one opportunity” to prove himself to the world of underground hip-hop.

Not so coincidentally, Eminem has the same number of chances to prove himself as an actor in the world of mainstream studio releases. And his ultimate, albeit not enormous, success as a thespian mirrors that of his success as an artist. His charisma draws you in, and a glimmer of underlying skills makes you stay.

The film opens with Rabbit nervously rehearsing rhythmic hand thrusts and mugging inflated machismo glares into broken mirrors as he prepares for an impending freestyle battle. His anxiety culminates in some serious puking (mom’s spaghetti?), and when finally holding the mic in the spotlight, Rabbit chokes and flees the club.

He lives at home with his mom (an incredibly underused Kim Basinger, “L.A. Confidential”) and baby sister (whose symbolic representation of Eminem’s own daughter is so painfully obvious, even remedial ninth-grade English students will catch on), has a rusty, busted-up ride and works long shifts at a car-part plant. It’s all been done before in the music-to-movie movie, but it is interesting to see the testosterone-fueled rapper take on such a loser of a character — and to do it so well.

Maybe too well. There’s nothing inherently likeable about Rabbit, whose screen appeal merely coasts on the fumes of Eminem’s own magnetism. He has a creepy devotion to his sister and is pretty funny when he wants to be.

Other than that, though, there’s not much to hold on to. Rabbit’s self-doubt, problems at home and issues of race are glazed over too quickly, are rarely addressed and are weak when they could have been compelling. There is no true turn in his morality but merely the glow of Hollywood cliché and the power of Eminem to let us know that he will win in the end.

This lack of character development is a common theme throughout “8 Mile.” Mekhi Phifer (“O”), who can be a force on screen, is somewhat tired in the role of Rabbit’s friend and is forever moving his ridiculously fake dreads out of his face — or not, which can be even more annoying. His lameness is only outdone by the usually haunting Brittany Murphy’s (“Don’t Say A Word”) sub-dimensional portrayal of Rabbit’s lust interest.

Eminem was smart to surround himself with such screen talents, but it is as if they all got a memo the first day of shooting that said, “Don’t out-act the rapper. He could more than likely have you killed.”

But their ethical loss is the Shady’s gain. While nothing spectacular, Eminem can hold his own onscreen for almost two hours. With cold, calculating eyes and an intensity in his voice, the emcee brings an authenticity to a story that was bound to be made sooner or later.

The natural drama of the underground hip-hop world of the mid-’90s makes for an intriguing backdrop, and, really, who better to play the lead than the man who embodies all the angst, stress and ultimate explosion of verbal creativity?

Eminem was also wise enough to involve himself with names like producer Brian Grazer (“A Beautiful Mind”) and director Curtis Hanson (“L.A. Confidential”). Hanson’s dreary Detroit is genuine, and when juxtaposed with the frenetic scenes of the hip-hop club and energy of the music as a whole, it brings to life Rabbit’s frustration and conflict of his potential versus his lot in life.

The final freestyle scene has all the makings of a great cinematic boxing match — the underdog pitted against the heavyweight favorite, the dark arena, the crowd, the sweat, the opponents exchanging verbal punches — Hanson and his handheld camera snowball what little narrative arch the film had into a solid, energetic climax.

In more than one way, this was Eminem’s “one shot, one opportunity” to flow in film. This was the part he was born to play. However, it’s fairly certain that, aside from a few cameos, he will never be in another movie again.

It is not so much his lack of acting prowess (that’s at least as on par as one-third of American actors today) but because there will never be another role for him. He can play the character of himself like no one else, and, despite what the hype says, “8 Mile” is clearly his life story.

All it really needed for success was the real Slim Shady, and it has it.