George MacKay practiced yoga 3-4 hours a day once he was cast as Bo so he could do the advanced poses he did in the film. He said it was the toughest part of the shoot for him.
At a screening in San Francisco, Matt Ross revealed that throughout filming, the children came to call Viggo Mortensen "Summer Dad"
The family's alternative lifestyle looks pretty realistic in the film - this is a detail Viggo Mortenson especially paid attention to and questioned Matt Ross to make sure all the living components were accurate. Luckily for Matt Ross, he drew on his childhood growing up with an alternative off-grid lifestyle in the Pacific Northwest, dissimilar from that in the film.
The actors went on a two-day survival trip and slept together as a family in a hut they built made of ferns. They learned how to track and start a fire with a bow drill, read a lot, and learned about the world's political systems.
Director Matt Ross had the actors who played the six fictional Cash children in CAPTAIN FANTASTIC sign a contract promising not to bring any electronic devices on set and to "keep processed foods and sugars to a minimum."