Let’s make one thing clear: A Prophet, which earned significant acclaim at the 2009 Cannes Film Festival (where it won the Grand Prix), does not reinvent the prison genre. Director Jacques Audiard (The Beat That My Heart Skipped), who co-wrote the script with Thomas Bidegain, adheres to timeworn conventions, including the inner rivalries, racial tensions, sudden spurts of violence and pervasive threat of being thrown in solitary confinement. At about 150 minutes, the picture periodically plods along, trying one’s patience.

And yet, thanks to Audiard’s instincts for casting and his willingness to get inside the head of his protagonist Malik El Djebena (Tahar Rahim), the movie maintains a raw power. Audiard incorporates a straightforward naturalistic aesthetic to introduce Malik, condemned to six years in lockup for beating a police officer. After he and we scope out the prison turf, understanding the divide between the Corsican and Arab populations, Malik is compelled to commit a murder, thereby making his loyalties clear.

Once the killing is carried out, Audiard starts to release himself from the formula’s shackles, as he finds the freedom to experiment with the picture’s form. Though Malik maneuvers through the prison, conniving and making deals with both sides, the picture plants itself inside his psychological space. With frequent digressions in that direction — his victim repeatedly emerges to torment him, deer and other apparitions offer a fantastical escape — what might be deemed “a portrait of the criminal as a young man” begins to emerge.

When Malik enters prison little is known about him. He has no family, friends or enemies. Rahim, in his first major role, adeptly buttons up, maintaining a stoic demeanor that’s punctuated with the bits of panic conveyed by his searching eyes. However, the longer he spends behind bars, as an apprentice to César Luciani (Niels Arestrup) and (at great risk) a friend to others, the more apparent it becomes that a new, confident insider is developing, with a unique personality taking shape.

The movie sustains itself with that micro focus. Rahim is in nearly every shot of every scene and he has the animal magnetism that compels one to be drawn to an actor for an extended period. He’s never less than utterly convincing, whether he’s the polished old-pro criminal, the withdrawn newcomer or somewhere in between. Audiard gives his actor the tools he needs to unlock the window into his head, incorporating an ideal amount of subtle, low-key quiet scenes to mix with the hubbub of all the criminal activity. It’s an ideal partnership that further cements Audiard’s reputation as an actor’s director.

While the details of the various schemes and negotiations are stretched to nearly interminable lengths, Rahim’s performance and the filmmaker’s propensity for incorporating abstract elements keep things interesting. The movie lacks the emotional scope of the best prison dramas and seems an inadequate metaphor for the hesitantly diversifying French society (where, for example, are the other prison minorities?). Yet this Oscar nominee for best foreign language film introduces a notable, wholly unforgettable new character and actor into the cinematic pantheon. That, in itself, makes A Prophet a worthy success.