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Much of “Captain Fantastic” takes place in the woods, with characters who live sustainably without running water or electricity. Authenticity was key, so director Matt Ross put in a call to survival instructor Jason Knight.

The setup: Ben (Viggo Mortensen) lives in the forest in the Pacific Northwest with his six children. But his way of life is tested after a family tragedy.

Practice what you preach: Knight, co-founder of Alderleaf Wilderness College in Monroe, Wash., worked with Ross during pre-production of the film (which bows July 8) to flesh out a curriculum for training the cast, then flew them out to participate in a densely packed tutorial. “We had to make sure all the needed survival skills were covered,” says Knight, who lives a mostly self-sufficient, wilderness-based life with his family on the Alderleaf campus.

Survival syllabus: Included in the two-day crash course were lessons on building a natural shelter, primitive water-purification methods, wildlife tracking, and making and using stone tools.

Best in class: Actors George MacKay, Samantha Isler, Annalise Basso, Shree Crooks, and Charlie Shotwell took the entire course. Mortensen made it for an evening. “George did great with the fire-making section,” Knight says. “Charlie surprised us with the amount of notes he took during the wild edible and medicinal plants section.” Adds Ross: “Both Annalise and Samantha particularly excelled at their butchering class.”