After I've watched today the movie Double Indemnity, I came to notice something: The greatest Femme Fatale is not the one that makes men do bad things, but the one who makes them want to do bad things. What the heck do I mean with that?
Femme Fatale is a kind of a character who uses her sexual "prowess" to control men and make them do really bad things, usually for her, usually for a promise from her, that she "forgets" to fulfil She tosses them aside, using the newer ones to eliminate the old ones among other more terrible things. Although this character originated in the French literature, she's more famous in the Film Noire genre.
Anyway, after explaining what it is, let's go to what she does in the movie, won't we? From the first moment we see her, she catches our hero's eye (can't say protagonist about a murderer) and starts to seduce him. She's all present, teasing him with canny answers to his questions.
The next time we see her, she becomes more of a "to the point" kind of girl. She asks about this accidents insurance policy. Neff immediately realises that she wants to kill her husband, to murder him, and says it won't work. He leaves, and later that evening there's a knock on his door. When he opens it, there's Phyllis again, and a short time afterwards is not only agreeing to sell her the policy, but he leads the way and plans how to kill, using every possible way to get more money.
Neff here is passionate about killing the husband, passionate about planning it, and most of all, doing it to get Phyllis (who has plans of her own). This last thing makes Phyllis the perfect Femme Fatale, she makes them want to do the bad stuff, and not just making them do it.
Applying it to RPGs is a little bit problematic, though. If one of the PCs is a Femme Fatale, going for this way of using it is not a problem at all, 'cause one can control the NPCs without making the others feel useless. On the other hand, Femme Fatale controlled by the GM can be an entirely different beast. Holding secrets from the players and their characters is the first step to making it easy. Making the character seem like a good person and her enemies like bad ones is the second stage. I like to play on my players thoughts about women and power, women and obedient and so forth against them. In this manner, it helps a lot when the players think that woman can't be nothing but the victims till it's too late.
Do you use Femme Fatales in your games? How do you use them?