Well, first off, JO is a much more serious person than Shaq and Pierce, and most of the other guys. In fact, I am not sure I have ever seen him smile. So I wouldn't read too much into that.
As for not being on the bench, there could be plenty of explanations, but most likely he was there, but may not have been sitting in an auxilary area, since he wasn't going to play. That happens a lot.