I think Jack was eyeing Ennis off from the start, when he was looking at him thru the rear view mirror, and I think he was attracted to him from then...physically.
At that time, when Ennis sneaked a peek at Jack, I think he was thinking "who's this dude, why is he looking at me like that?"
Within days I think they had developed a mutual respect for each other, with both of them working together as hard as each other. And as has been said before, they listened to each other and both showed an interest in each other's past. I think Ennis felt comfortable with Jack, as is shown by how much he opened up to Jack, something he had obviously not felt before.
The first time I noticed an intimate attraction from Jack, was after Ennis fell off the horse, and the gentle way, Jack wiped his face, it was more than just a buddy type thing, it was something more, and I think Ennis noticed it too, thats why he grabbed the cloth from him.
I think Ennis was maybe noticing that the attraction Jack had for him, was a little bit more than what is usual for a couple of blokes to have, but I dont really think Ennis was looking or feeling that way towards Jack, he felt comfortable, he felt relaxed, he lked Jack's company, probably better than any other man he had kept company with, but I do not think the feeling was attraction and definately not, sexual attraction.
That did not happen until after TS1......when he rode up to the mountain, and gave himself space to think about what had happened, and exactly what he was feeling. I really dont think it was until that time on the mountain, when they had the "I aint queer" conversation, that Ennis felt more of an attraction to Jack, and from then it just got stronger and stronger.