“I need just a simple favor,” Blake Lively tells Anna Kendrick in a new teaser for “A Simple Favor.”  “Anything,” says Stephanie (Kendrick), clearly awestruck by her “wonderful, elegant” new best friend Emily (Lively) and eager to please her. Given the footage that follows, it seems that Emily’s request was anything but “simple.”

We see all-too-brief glimpses of police showing up at Stephanie’s door, lots of crying, and guns. Emily is missing — and she’s been living a double life. “People do terrible things for their own reasons,” she says. But Stephanie has secrets of her own, and her apparent obsession with Emily is mighty suspicious.

“A Simple Favor” is based on Darcey Bell’s 2017 debut novel of the same name.

Last seen in “Pitch Perfect 3,” Kendrick’s upcoming projects include “Noelle,” a fantasy comedy that sees her playing Santa’s daughter, and “Trolls 2.”

“All I See Is You” and “The Shallows” are among Lively’s recent credits. The “Gossip Girl” alumna is signed on to topline another adaptation, “The Husband’s Secret,” based on the book by “Big Little Lies” author Liane Moriarty.

Co-written and directed by Paul Feig (“Spy,” “Bridesmaids”), “A Simple Favor” hits theaters September 14. Jessica Sharzer (“American Horror Story,” “Speak”) penned the script with Feig.