She went from sweet and caring to crazy real quick. That's how I picture Leah most of the time.
Well, I guess I just disagree. I picture Leah as both. At the same time. The only time she's really had "mood swings" was when she was locked in Arctis Tor. All other times she comes off as acting very normal about being very... not normal. She actually doesn't
act crazy, she acts cold. She is, most often, alluring in all respects, with nothing in her behavior really being a tip-off for how dangerous she is, but that danger is there all the same. Peril simmers in the air around her, but is easy to miss or ignore because she looks kind, beautiful, flawless, and even serene. Poisoned nectar that seduces mortals to their doom, without setting off any alarm bells in the unwary.
Maybe Firefly Christina rather than Mad Men Christina.
Well, yeah, of course. It was her performance on Firefly that was the basis of how I picture Leah behaving. I can't imagine that her skills have fled her in the years since then, though, especially since I know Mad Men has generated some level of acclaim (I haven't watched it.)
I picked her because I think she could pull off the sweetly venomous character of the Leanansidhe.
I love that you're using my words for the character and everything, but again I ask, "Why?" What indications has she ever given that she is able to pull off a character who is menacing and inviting
at the same time? Because not many actors can do that, and I've certainly never seen Ms. Janssen do it. She does menacing well enough, but it's very clearly menacing in a "run the other direction" sort of way, rather than a "wow, I kind of want to kneel at her feet even though I should probably be running the other direction" sort of way. Harry basically feels that way around Leah, even though he absolutely knows better, so it must be some pretty powerful appeal. I'm just not convinced that Famke Janssen has that, based on what I've seen of her. Christina Hendricks, on the other hand, I would have some trouble with wanting to run away from, even when she's clearly out to cause some harm. She can make a character seem innocent and seductive when she's at her most dangerous.