Captain Fantastic, then, is a story about culture shock - and the shock waves that threaten to break up a close-knit, loving clan. Mortensen is just about perfect here.
One minute he's exclaiming "Stick it to the man!" - delighting his kids as they outsmart cops and supermarket managers. The next, he's strumming a guitar in a campfire jam. Or facing off against his angry, affluent father-in-law. Or stopping to consider that his strident child-rearing philosophy might have been a mistake.
But if this is Mortensen's movie, he shares it with an amazingly adept group of young actors. From McKay (conveying a keen intellect and an aching shyness) on down to the tiny Shotwell, Ross' ensemble of pip-squeaks, preteens, and adolescents works in sync, and in competition with each other, just like real siblings. How Ross cast this gang, and how he got these very different kids to coalesce, well, it's a feat.