Spontaneity and going with the flow are great, but there are certain times when talking with your partner about what will happen or setting rules (like safe words, which you can read about here) is necessary. For example, if something calls for punishment, degradation, humiliation, or simulated lack of consent, you guys should discuss boundaries and how you'll communicate during and after, and talk through a bit of what might happen.
That said, just because you set parameters ahead of time doesn't mean that plan can't change. "Having a safe word, for example, is not an excuse for not checking in with a partner, being in tune with your partner, and recognizing what’s happening with your partner during sex," says Kerner.
This is especially true if you get into BDSM territory, which can have a lot of overlap with role-playing. More info on how to navigate that here.