How does arn mange to "stay invisaible"?

Arn manages to be invisible by avoiding interaction as much as he can, and by doing things the Khmer Rouge would like. For instance, when in line for the Khmer Rouge to check people's skin color, Arn rubs dirt on his face so he will look darker like one of them. That kept him a bit more invisible and alive for another day. He also uses this trick to make sure people who know him do not interact with him. A personal discussion could also lead to your death.