From the Dazed article by Nick Chen:
‘Will and 13-year-old Tom, a father and daughter camping out in the wilderness, are what most of us consider to be homeless. The self-sufficient duo, played by Ben Foster and Thomasin McKenzie in Debra Granik’s poignant, socially conscious drama Leave No Trace, live off the grid in a park in Portland, Oregon. They forage for food, squeeze drinking water from moss, and sleep in a tent surrounded by howling wolves. When Will isn’t “home”-schooling Tom, they’re playing chess or practicing drills to avoid detection. No phones, no address. To the outside world, they’re practically invisible.
…preparation for the film involved immersive training for the cast and crew with Nicole Apelian, “a very masterful forest survivor” according to Granik.’