You are a movie director needing to capture child actors at their most child-like and real. What do you do?
Matt Ross, the helmer and writer of the Cannes-honoured drama- comedy Captain Fantastic, has a piece of advice: "Never turn off the camera."
You might get priceless moments of authenticity, such as at the start of Captain Fantastic, in a segment showing the forest-dwelling, back- to-nature Cash family making music around a campfire.
While filming the scene, Ross saw that the kindergarten-age Charlie Shotwell, playing the youngest Cash child Nai, felt relaxed enough to do what kids do: pick his nose.
"Charlie did not pick his nose because he thought it would be funny. He literally forgot we were filming him and his nose itched and he picked it, and we captured it," he tells The Straits Times on the telephone from New York.
Captain Fantastic, which stars Danish-American actor Viggo Mortensen as family patriarch Ben Cash, netted Ross the directing prize at the Un Certain Regard section of the Cannes International Film Festival this year. It is now showing in cinemas here.
Ross created the story of the Cash family right after he became a father and realised that to be the best dad he could, he would have to shield his children from everything wrong with modern society - processed food, trash culture, peer pressure and electronic screens.
In the film, Ben and his wife Leslie (Trin Miller) raise six children in the woods of the Pacific Northwest, without even a telephone connection. The brood hunt and plant their own food, read the classics, make their own music and train their bodies like athletes.
But the peace is shattered by an emergency that forces them to leave the haven for the larger America, described by Ben as "under- educated and over-medicated".
Mortensen, best known for playing Aragorn in The Lord Of The Rings trilogy (2001-2003), contributed much to shaping the character of Ben.
In the original script, Ben is "a little more playful, a little more jokey with the kids, played by an actor with a twinkle in his eye", says Ross, 46.
But Mortensen, 57, opted to take a different path. His Ben is stoic and serious, carrying "a gravitas".
"I loved his interpretation, he made it better," says Ross.
Ross himself is a veteran character actor, now appearing in the HBO comedy series Silicon Valley, in which he is Gavin Belson, a tech tycoon with a hair-trigger temper.
He wrote the part of Ben with Mortensen in mind, seeing him as that rare actor who could embody Ben's mix of rugged manliness and articulate idealism.
"And that actor had to be American. There are some Australian and British actors who have it, but in terms of American actors in the age range, I couldn't think of anyone else," says Ross.
The film also stars stage and screen veteran Frank Langella as Ben's rock-solid conservative father- in-law, Jack.
Ross sent a box of books to Mortensen, so he could better grasp the political bent of his character. The reading list leaned heavily towards the American left, including such writers as Howard Zinn and Noam Chomsky.
In the film, the Cash family celebrate Chomsky's birthday instead of Christmas. That idea comes from Ross' own life - his family celebrates Noam Chomsky Day, with cake and readings from his books.
But as it turned out, Mortensen was already acquainted with the works of these intellectuals, says Ross. "Viggo's a very well-read man," he says.
• Captain Fantastic is showing in cinemas.