hrm... sometimes it really gets me off if Martel and I pretend i'm some hot college co-ed who has come over to study and ends up getting fucked... It can get us both really really hot, but sometimes i'll "break character" just to have some reassurance from Martel. I'd suggest sitting down with her and talking about what you hope to get out of role playing. Role playing is kinda like BDSM in that you're entering into a contract with the other person. If you get serious into it, it's like you're both acting out the parts of other people, and you need to know just how far you want it to go. For example, if she's role playing a naughty teacher and you're her teenage student, and she decides that you need a spanking with a ruler, you need to come up with a safe word in order to tell her that you dont want to do that.
Nothing as strenuous as a stop word as in BDSM, but perhaps a cue word or phrase that would let you both know, without breaking character, that you're not comfortable with what the other person is doing. For example, if she said "Oh, you've been a naughty boy, now I have to spank you!" you could say something like "Oh, but I much prefer xylophones!" and that's the cue phrase to let her know you're not cool with the spanking.
COMMUNICATION IS KEY- IN EVERYTHING!

__________________
Sage knows our mythic history, King Arthur's and Sir Caradoc's
She answers hard acrostics, has a pretty taste for paradox
She quotes in elegiacs all the crimes of Heliogabalus
In conics she can floor peculiarities parabolous
-C'hi