You don’t realize how much a good horror movie depends on acting until you stumble in that rare one whose cast actually gets it right.

We need to believe that the people up there on the screen  are shocked at what they’re seeing, mortified at their mortal danger. And in “Lights Out,” we do.

It’s an elemental thriller, a ghost story told in pools of light in deep darkness, with a child in peril and that moment when the child recognizes that crazy Mommy (Maria Bello) isn’t just talking to herself and won’t be any help at all with this boogey-girl haunting his nights.

“Did we wake you?”

And that little boy, Martin, played by Gabriel Bareman? He plays that kid scared out of his wits, shutting his door, afraid to turn the lights out. Because that’s when “Diana” will get him.

Teresa Palmer of “Warm Bodies” and “The Choice” plays Rebecca, the Goth half-sister who left home, it turns out, for the same reasons Martin comes knocking at her door. She has faint memories of being menaced by Diana. But can she keep Martin safe in her tiny apartment upstairs from a tattoo parlor? Will the pink neon “Tatoos” sign shining through the windows be enough to keep Diana at bay?

David F. Sandberg, remaking his own short film and working under horror impresario producer James Wan (“Insidious”) sets up the terror, the monster and the dilemma — nobody believes them — nicely. An opening scene puts Billie Burke (“Twilight”) in jeopardy, the missing “Dad” in all this, jump-starting a brisk 81 minute fright.

The magical talismans, if you know your “Morphology of a Folktale” ingredients, are well chosen — a hand-cranked generator/flashlight, candles, a cell-phone scene. Sound effects and jolts of music heighten the effects.

“Lights Out” loses its way at times. Things are over-explained, background material we might be curious about is fleshed in needlessly. Yes, it would have been a 75 minute movie (almost unheard of these days) without this filler. Does it really matter where this night terror came from?

But Bello, Bateman and Palmer more than give us fair value as their frights become out frights, and their dilemma has us wondering how bright out cell-phone light is, wondering if the flashlight in the glove compartment still has batteries that work, wondering how long we can go without sleeping.

MPAA Rating:PG-13 for terror throughout, violence including disturbing images, some thematic material and brief drug content

Cast: Teresa Palmer, Gabriel Bateman, Maria Bello
Credits: Directed by David F. Sandberg, script by Eric Heisserer (screenplay), David F. Sandberg. A New Line/Warner Brothers release.

Running time: 1:21