I have noticed it too,but I have never attributed it to some Heath's lapsus...Not only because he had a great degree of professionalism,but because Ang is a perfectionist;enough as to make them repeat this scene several times,so I don't think he wanted to leave any kind of lapsus in the final result.Is it a little "un Ennis",as you say?.Yes,it is,but,maybe is not the whole scene in this way too?.I think it's one of the few times we can see a "bold" Ennis able to let himself be carried for his wishes and impulses.His face is rather mischievous,maybe only full with lust,not only when he looks around,but during the instants he takes Jack against the wall.For me,it's the face of a man who has before him his wishes object,the man he has been looking and longing for during four years,so he's more than overwhelmed with emotion...and some kind of emotions.In any case,not a Heath's fault,even if due to the times they had to repeat an awkward situation for both of them.