It occurs to me that Jack doesn't think much of Alma. That's why he's so flippant about criticizing her when they are out at the fire, "You and Alma? That's a life?" AND why he doesn't understand why Ennis won't just up and leave her. Maybe that dislike of her is part of that stare. She's an obstacle between him and Ennis.
I think you're right about Jack not liking Alma. It's only natural, considering she has what Jack wants more than anything -- Ennis.
This scene is my favourite, and it is so powerful in so many ways. The passion of the reunion makes me gasp for air every time I see it (and I'm up to about 15 viewings, so I shouldn't be surprised anymore...). The relief in Jack when he realizes that Ennis has longed for this as much as he has. The pain it causes Alma when she sees her husband lose control like that - and doing it with another man. And I think you can see on Ennis' face that he knows he is hurting her, but he can't make himself care about it, because Jack is all that matters.
Everytime I watch BBM I walk out with an aching stomach and a slight headache, because I can't stop myself from feeling everything the characters in the movie are feeling, and the aching ALWAYS starts at the moment when Alma is looking out the door. She's dressed up in nice clothes, and she's curious about her husband's friend - probably because Ennis usually doesn't bring friends home. And what she finds outside her door is so completely foreign to her, that she can't seem to grasp it.
The marvellous acting by all actors in this movie is just one of the (million reasons) why I love BBM. What's so great about it is that you don't really see Heath or Jake acting - you see Ennis and Jack. Period. (BAFTA got it right.
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