In Thich Nhat Hanh's tradition, part of meditation posture is assuming the half-smile. Even though that is not taught in the Tibetan system in which I'm trained, I try to do it from time to time. It's amazing how it shifts one's attitude:
I don't think he's suggesting that we not take action. Thich Nhat Hanh is a person of action himself. But he's saying it needs to start with oneself. We need to be the change we wish to see.
Smiling is very important. If we are not able to smile, then the world will not have peace. It is not by going out for a demonstration against nuclear missiles that we can bring about peace. It is with our capacity of smiling, breathing, and being peace that we can make peace.