The Sergeant's Lady has two main baddies: Anna's first husband, Sebastian Arrington, and a young lieutenant in Will's Rifle company named George Montmorency.
Sebastian is a tall, dazzlingly handsome blond cavalry officer. Kind of like Ralph Fiennes, if you imagine him without that lovely smile and playing the part of a misogynistic control freak.
Montmorency is tougher to cast, since he's meant to be the sort of person who fades into the background. Best I can do is David Tennant...but he's far too adorable, and I was never a Tennant fan! (Christopher Eccleston was and remains my Doctor.) Anyway, if you hired him to play a bitter young man with a massive sense of entitlement, I'm sure he could ditch the charm and sweetness and make a good Montmorency.