I was j/s in coach the other day and there was another pilot in the back. We were both off line jumpseaters. Well while we are taxing the guy saw a open seat in first and just got up and moved up there without asking or anything. The FA said nothing to him. We checked in at the same time with the gate and the captain. I was in awe, not only at what he did, but that the FA did not say anything to him. Seemed like a good way to lose the j/s privilege for the rest of the pilots at your airline to me.