“Ah don’t have to remind you that her daddy thinks he is the ‘Jesus J. Edgar Christ’ of the Appalachian Mountains,” the mountaineer crime patriarch drawls in warning to his only son, who has taken up with a local prosecutor’s “rabbit-ass daughter.”

“Mah daddy, his his daddy before’im, and now you — we do not choose this way of life,” he tells his boy, Jacob (Hopper Penn). It chose us.”

There is only one Thornton, and he goes by “Billy Bob.” In “Devil’s Peak,” the film adaptation of David Joy’s “Where all Light Tends to Go,” Billy Boy Thornton sinks his teeth into Southernspeak for a modern Southern gothic tale of “outlawing” as the family business, which these days and in these foothills and hollows means “meth.”

Thornton is the colorful linchpin of this somber, slow but simmering crime melodrama, a film that also features Jackie Earle Haley as the comfy-with-looking-the-other-way sheriff, and Robin Wright as the ex-wife Charlie McNeely (Thornton) gave up on when she got hooked on the drug his clan now sells all over Jackson County, N.C.

Penn plays the 20ish lad still figuring out his family’s criminal legacy and how he fits in it, and how he can hang onto the girlfriend (Katelyn Nacon) his daddy disapproves of as he faces the ugly, illegal responsibilities that come with being a McNeely.

Director Ben Young and screenwriter Robert Knott lose themselves in Joy’s colorful Appalachian colloquialisms and Thornton’s peerless way with such lines.

“If this thang needs to’go off for some reason,” Charlie says as he hands his kid a revolver, “it touches mud and water. Got that?”

Got it.

Thornton’s menace, augmented by a Satanic dyed goatee and bald pate, mixes easily with the folksy way he has of relating Charlie’s family anecdotes, each and every one designed to instruct via a life lesson learned, almost all of those lessons chilling. Charlie, we can guess, can be utterly heartless when the need arises.

Haley’s county sheriff is something of an archetype — casually corrupt, but seemingly level-headed, with hints of compassion.

Wright makes a decent impression in a limited number of scenes, as does Emma Booth, brassy as the bald old crook’s half-his-age-hussy.

“Devil’s Peak” is a simple story whose filmmakers lose track of threads and characters, perhaps owing to editing. Jacob’s devotion to his girlfriend is thinly-developed, her politically-ambitious stepdad (Brian d’Arcy James) practically an afterthought. I am predisposed to go for Appalachian stories, so some of that I let slide.

A bigger issue is how this mixed-bag thriller is Exhibit A in the whole debate over Hollywood “nepo babies,” all those celebrity offspring who follow their parents into “the family business,” and how that often doesn’t pay dividends on the screen.

Hopper Penn is the son of Sean Penn and Robin Wright, and even playing callow lad of 20ish, he’s never more than adequate in the part and not always that. But without him, Mom doesn’t sign on, the film doesn’t have three big name stars to ensure its value and get financed and “Where All Light Tends to Go” isn’t adapted for the screen.

The Catch-22 of casting meant that they compromised on their lead just to get the film made. It happens all the time.

They had the makings of a solid, gritty and distinctly Southern B-picture. But their young lead, without whom I dare say this never would have been made, has an arresting look and yet little screen presence or acting craft (no acting school for him) to compensate for that.

Rating: unrated, violence, drug abuse, profanity

Cast: Billy Bob Thornton, Robin Wright, Hopper Penn, Jackie Earle Haley, Katelyn Nacon, Brian d’Arcy James and Emma Booth

Credits: Directed by Ben Young, scripted by Robert Knott, based on a novel “Where all Light Tends to Go,” by David Joy. A Screen Media release.

Running time: 1:35