Yes Marie, I think you hit the nail on the head there......it was Jack who was flirting, and he was looking for any sign at all that maybe Ennis was catching on to that fact, and maybe even flirting back, even just a little bit.
I think Ennis felt comfortable with Jack right from the start, and that comfort grew more and more every day. And, not only did he feel it, he also felt how comfortable Jack was with him. And I dont think Ennis had felt like that before, with anyone, especially a stranger who he had just met.
They seem to have a similar sense of humour, and they could laugh and talk with one another, and I think Ennis was probably thinking..."This is good, I like being with this guy".....he felt good and relaxed.
I think he was probably flirting humourously, rather than flirting sexually, and maybe somewhere he and Jack realized that it had unconciously moved from being humourous to sexual, and in there own secret way, were enjoying that fact.
I think in TS1, when Jack groped Ennis, originally it was just something he did in his sleep, but once Ennis woke up, and they looked at one another, it was like, "OK, we've been playing around with each other verbally, come on, lets do the real thing".......
The rest is history................