I suspect that in FNIT, Jack was a bit taken by surprise at being flipped. Personally, I interpreted his unbuckling as a sign of an initial intention to play a more active role. Just my opinion, of course...
But in the SNIT, when Ennis enters the tent, it is the opposite -- Jack pretty much knew that Ennis would give in, and so he was more confident and better prepared for what was to follow. His more active role in SNIT is worthy of note...
Yes,me too...But I tend to interpret his unbuckling as a sign of "playing" a little more before; I see this the more active role as a way of behaving himself more in SNIT line,not in a sexual act itself necessarily.
Jack had the opportunity to get up and refuse Ennis in FNIT. But he didn't. In fact he was waiting impatiently for Ennis...

I doubt that he ever hoped for Ennis to play a passive role. In fact, as I see it there were no defined expectations in FNIT except of sexual enjoyment, one way or the other. Jack had initiated things and waited for the answer, ready to take any answer from Ennis. I don't think that he would expect Ennis being so closed and stiffed, could pay a passive role there.
SNIT is a different story and once MPJ said that she could see Ennis in passive role there, as entering the tent he already surrendered. Well, I have to admit that maybe I could consider this possibility. Maybe...

Ennis was too sexy when entering the tent, for the record!
