Jacques Audiard’s “A Prophet” is a long, richly detailed prison opus that spans the six-year sentence of Malik El Djebena (Tahar Rahim), a young Arab man imprisoned for attacking a police officer. The film, which won the Grand Prize at the 2009 Cannes Film Festival, is at its core a faithful entry into the well-worn lockup genre, rife with entangling alliances and brutal violence. It periodically drags during its stretched thin 150 minutes and lacks an emotional payoff. Yet newcomer Rahim gives an intense, galvanizing performance; it’s the sort of work that convinces you an actor’s tapped into a character’s soul. To supplement his star, Audiard smartly transcends the genre’s trappings, creating a strong interior sense with his visual renditions of Malik’s headspace.