I already spoke of Jack's open eyes expressing his love. What I found as Jake/Jack's peak of his character was how in some situation, when he wants to hide his emotions, he put his head down. The hat covers his face and he's safe from exposing himself completely.
He does it even when no one looks, because he is obviously aware of his honesty and sincerity, and it's that way that he protect himself form the world.
1. after FNIT, when Ennis comes and says: "this is only one time ....", Jack just put his head down to reply: "Nobodies business but ours." No one is there to watch, but his feelings are all around and he needs to hide them.
2. When he ask Ennis if he would come again next year to Brokeback before they depart, his eyes are so vulnerable that he has to put his head down, knowing that he's already been too open to Ennis.
3. After the reunion kiss, he's trying to be polite with Alma, speaks some words about his wife and son, but also can not bear the closeness of Ennis's body, so he just put his head down.
I think that there are also some other scenes, but these 3 I remember the most.

Jake did a tremendous job here! Ave Jake! (

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