Originally Posted by WMR
Condensing is way different than complete deviation. There was no good reason to sub "talisa" for Westerling, IMO. They could have even kept the more sensational way of dealing with her but kept more loyalty to the source material.
I think they replaced her in an effort to 'condense' the convoluted Westerling subplot out of existence. I always felt it was a little too convenient that Robb fell in love with the daughter of a Lannister bannerman. By bringing in an impartial love interest from Essos, they eliminated one of the reasons to be skeptical of their relationship.