Here are some things you might expect to find on HBO: Prohibition-era gangsters. Post-Katrina New Orleans jazzmen. Vampires. Bill Maher.

Here’s something you might not expect to find: an intense ninety-minute closed-room debate about the existence of God, the authority of the Bible, the meaning of suffering, and the nature of salvation.

You’ll find just such an impassioned religious confab in The Sunset Limited, a Cormac McCarthy–scripted drama making its screen debut on HBO on Saturday, February 12. Directed by Tommy Lee Jones, and starring Jones and Samuel L. Jackson, this heartfelt (if creaky) two-person film conjures up an agonized encounter between a nihilistic university professor (Jones) and the devout Christian ex-con (Jackson) who has just stopped the academic’s suicide attempt.

In the former convict’s New York slum apartment, the Jackson character (designated in the credits as Black) tries valiantly to win the pessimistic professor (White) to faith. Bantering, bullying, asking questions, telling jokes, and relating a bloody jailhouse anecdote, Black strives to connect with White while poking holes in his logic. White gloomily concedes that his own worldview’s secular-humanist underpinnings have turned out to be “very frail, very fragile,” but he clings to his atheism and despair. It’s a standoff. “What am I, a prisoner here?” White demands, when Black declines to open the apartment’s locked front door. “You were a prisoner before you got here,” the ex-con retorts.

The bleak and claustrophobic setting—sickly green walls, chained and dead-bolted front door, crucifix hanging beside a roll of paper towels—reinforces the mood of white-knuckle existential grimness. The static environment also testifies to the film’s theatrical roots: The Sunset Limited was originally a play that premiered at Chicago’s renowned Steppenwolf Theatre Company in 2006. (It later had an off-Broadway run in New York.)

On the basis of this adaptation, which hews closely to the play, one might urge novelist McCarthy, author of The Road and All the Pretty Horses, among other fictions, not to quit his day job. An irritating staginess clings to Sunset’s philosophically freighted debate, with its ping-pong rhythms; its strategically positioned snips of exposition; and its digressions, repetitions, and misunderstandings gathering their own talky momentum. Director Jones keeps the dialogue clipping along at a satisfying pace, but his attempt to pep up the visuals by filming from a variety of angles is more conspicuous than effective. (HBO’s publicity materials relate that McCarthy himself volunteered engineering know-how to help the film’s production designer with the moveable wall and ceiling panels that allowed for roaming cameras. The famously reclusive novelist—a one-time mechanic and amateur architect who was raised Catholic—visited the film’s twelve-day shoot in Santa Fe.)

Its limitations notwithstanding, Sunset has moving and eloquent moments, and it certainly provides plum roles for its stars. Jones (who headlined the movie No Country for Old Men, based on the McCarthy novel) is a picture of numbed hopelessness: His eyes are pain-glazed, his unshaven face scored with wrinkles, his tone world-weary and detached.

By contrast, Jackson gives his character a winning charisma, with a jocular manner quickened with bursts of muscular earnestness. In a moment he can shift from good-humored teasing to gravity: When White, at one point, snaps, “I’m sorry, but to me this whole idea of God is just a load of crap!” the ex-con turns suddenly clownish, clasping his hands together, and drawling, “Lawd have mercy! Jesus help us! The professor done gone and blasphemed all over us! Well, he never gonna be saved now!” Then he stops goofing, and suggests, with wry matter-of-factness, that God isn’t petty enough to take umbrage at such human disrespect: “The Man Upstairs...done heard it so much that it don’t bother him too much.”

The battle of beliefs rages with such intensity that the story’s thin veneer of realism sometimes slips. Could Black be an angel, sent to lure White to salvation? Or—a possibility that seems more likely as the film goes on—is White a test of Black’s faith? If so, it could be a test that will trip up the Christian. When dawn comes after the two men’s night of arguing, and the white sun widens to fill the entire frame, the light might signal God’s presence—or it could be the glare of emptiness. Cormac McCarthy isn’t letting on.