From the LA Times article authored by Debra Grank:
Nicole Apelian, the wilderness educator and guide, read the script and gave us extensive notes on how people like Will and Tom might go about their daily activities. Ben Foster and Thomasin McKenzie, who play Will and Tom, began their acclimation and rehearsal process learning wilderness survival skills from Nicole, and they developed their strong on-screen bond through this hands-on learning process. That connection is not something that I can write. It is enacted, felt, it comes through looks and actions as the actors do real tasks and experience each other in an immersive setting.
Read the full article at the LA Times.