“Frailty,” Bill Paxton’s directorial debut, provides filmmakers with a study in engaging an audience. Working within the boundaries of a familiar genre, Paxton crafts a spooky, slow-moving mystery designed to draw in a crowd and keep them eagerly watching. While the film does not break any new ground, its sense of suspense and self-confidence propel it beyond expectations.

Paxton and writer Brent Hanley peel the layers of their story deliberately, transporting the audience to 1979 through the eyes of confused, confessing Fenton Meiks (Matthew McConaughey). Convinced that his brother Adam is a serial killer who calls himself God’s Hands, Fenton relates his Gothic tale to skeptic FBI Agent Wesley Doyle (Powers Booth), beginning with a portrait of a happy Southern family.

Paxton plays Fenton’s father, who raises his motherless boys with love and a foreboding, muted cheerfulness, with humility and simplicity . As the boys sleep, their father receives a vision from God, asking him to collect “special weapons” and destroy demons who have disguised themselves as humans. While Fenton is vaguely aware that his father has been struck by disease, Adam happily follows in his “superhero” father’s footsteps.

After an angel reveals a list of names to the father, the Meiks family begins to enact its own justice, burying the bodies in their rose garden. Young Fenton (Matthew O’Leary), torn between loyalty to his family and sympathy for their innocent victims, fights urges to confess and run away.

Fenton’s sense of filial piety is central to the film’s tension. Paxton carefully creates a family drama in the midst of a murder mystery, and the choice is a wise one. “Frailty” may easily have been a cheap horror movie if it had focused solely on the crazed God’s Hands, or an ordinary mystery were it told from the point of view of the FBI.

Instead, “Frailty” shows the bonds of family to stretch in accommodating horror; in the end, they strain and fracture under the pressure. Adam remains his father’s follower throughout, and even Fenton obeys fully, stopping short of only murder.

The period of Fenton’s obedience is where “Frailty” excels. The Meiks family is calm and quiet with their transgressions, maintaining a sense of normalcy. Paxton refrains from making the father a fanatic; instead, he is soft-spoken and persistant, rarely mentioning religious dogma and offering tried-and-true, pithy statements of fatherly advice.

In one scene, when Adam tries to convince his father that the school bully is a demon, his father admonishes him and adds “God will give you visions when you’re older.”

O’Leary and Jeremy Sumpter, who plays young Adam, are suitably doe-eyed but often fail to muster enough emotional complexity. Whether it is due to limited acting abilities or intentional direction is uncertain, but the child actors seem overly intent on playing up the family drama and playing down the horror. McConaughey is just downright spooky, slowing his drawl to a sluggish pace, like an older kid telling a ghost story at camp.

But the other elements of the film maintain an adept balance between drama and horror, adding depth to what would otherwise be a typical genre picture. Cinematographer Bill Butler carefully assembles contrasts between bright small-town sunlight and dank fatal cellars in the flashback sequences. The present-day scenes, however, are lit a bit melodramatically, as if the movie were a shadier version of “The X-Files.”

Thankfully, both Butler and Paxton largely avoid horror-movie cliches, although they cannot forgo a few moments of disturbing sound effects and spattering blood. But Paxton also knows how to use long silences to unsettle viewers, proving Alfred Hitchcock’s adage that suspense is always preferable to surprise.

The only true surprise in the film — not counting a few grotesque murder scenes — is a sudden, confounding plot twist. While it is certainly welcome, the twist highlights a few inconsistencies in the plot and also fails to resolve anything. Paxton gets caught up in the Hollywood trend of mistaken identity and tricks of the mind.

These small flaws, however, do not detract from the overall effect of “Frailty,” which is to demonstrate and destroy the powerful bonds of family against a genre-specific backdrop of murder and mystery. Paxton seems to have a specific purpose in mind — from soft acting and precise visual effects — and he fulfills it.