Posted by: @kingedthejust

Why do you think they do not talk to each other much?

I guess because they don't talk much in the books, come to think of it. Edmund doesn't tease her like he does Lucy or argue with her like he does with Peter. There's not much you can dramatically do with him and Susan. He just generally resents her like he resents all his siblings.

Posted by: @kingedthejust

Would the movie be better had they had more lines together?

Not really. I mean I think relationships between different characters can be interesting but the movie and the book already have a lot of well developed relationships like that. (There's Peter and Lucy. Susan and Lucy. Peter and Susan.) I agree with your suggestion that more dialogue between then was unnecessary. 

It is true that Susan's line in the movie (when she meets Aslan) that she was hard on Edmund too is incorrect. Actually, she was the only one who wasn't hard on him. But I can interpret it as a charitable lie on Susan's part even if I'm not sure that was writers' intention.

I agree that there isn't time in the stories, and certainly not the movies, to develop a relationship between these two. 

I  know Susan argued with Lucy & Peter in PC, and Edmund supported Lucy in it. If the older two had returned to Narnia in the third story, we might have seen something between them.  In the end, we learn that Susan no longer gets on with her siblings, due to her idea of being grown up. 

Susan the Gentle and Edmund the Just may not have had so much in common even though they were siblings.  Their relationship was perhaps a little distant but mostly accepting and tolerant. There wasn’t much conflict between them even when they were very young.  Susan didn’t seem to get angry or upset at Edmund like Peter and Lucy did at Edmund’s lying about the existence of Narnia (his saying it was all a game). Forgiving Edmund was not so difficult for her (perhaps much easier than for her siblings). She was gentle as was her descriptive name. I don’t get the feeling that Edmund and Susan ever disliked each other.

I just remembered there's a long scene with Edmund and Susan in The Horse and his Boy. If Walden Media had been able to adapt that book, I'm sure it'd have been included. 

The writers of the script probably just didn't think too much about it.  Susan never really seemed to have all that much to do in the books until HaHB where she was a major plot device.  Her desire to be grown up in a worldly way was needing to be tempered by Edmund who had become known for his wise counsel. And she was still amenable to it at that point.  But we don't see that sort of relationship beforehand.  I

n the LWW and PC movies the writers had to juggle all these different relationships and character arcs that it was probably difficult to come up with Susan and Edmund elements that would both serve the plot and add to their characters.  But from a realistic perspective, it does seem a little weird that they don't have those moments.  Peter always had a soft spot for Lucy so it would seem to follow that Susan would have taken Edmund's side on things.  She sort of echoes some of Edmund's skepticism early on in LWW (talking about the movie) but she doesn't really actively interact with him or even actively stand by him with a "I agree with Ed on this" or anything like that.