Director Jeff Wadlow says “The Devil’s Mouth” is the hardest film he has made, combining shark-movie suspense with claustrophobic, one-location horror. The story follows two college friends, played by Kathryn Newton and Lana Condor, whose relationship is tested when their group becomes trapped in a Thai freshwater cave with a bull shark; Wadlow says the film also explores female friendship in a more nuanced way than the genre often does.

The screenplay, by Aja Gabel and Myung Joh Wesner, appeared on the 2019 Black List, and Wadlow rewrote it after being drawn to its cave setting. Newton, Condor, Gavin Casalegno and Tommi Rose trained for two weeks in treading water, breath-holding and kayaking, and performed most of their own underwater, climbing and diving stunts; one 35-foot cliff jump was handled differently. Production built 13 sets across two soundstages to create the illusion of an extensive cave system.