I agree with SurrealObsession, and this is a fantasy of mine too, but I wouldn't REALLY want it to happen. For me, for example, what turns me on is being held down, struggling a little, and being talked dirty to in a way that makes it feel more non-consentual. That might be pretty vague but hopefully that helps. I'm pretty sure her mental image is more "violent" than what she'd actually want, so take it slow and ask her specific questions about what she'd like as the other post suggests. Or, just try a few "soft" non-consent type things like, sneaking up behind her and covering her mouth with your hand while you feel her up from behind and tell her "shut up or else", something like that. See how that feels to her, and go from there. Hope you guys have fun with this!
"I haven't been fucked like that since grade school."
-Marla Singer, Fight Club