Like many others have mentioned before, something that always enhances the experience for me is sensory detail. This can apply not just to the role playing, but the scene setting too. Even a minute comment about sight, sound, smell, touch, or taste can really tease the imagination 
My rule of thumb is to be safe, and persuade by suggestion than dissuade by force. I like to disclose that I love a good role play, and I want my partner to get as much pleasure out of it as I do. There are as many different boundaries and thresholds as there are people, after all.
Apart from that, just get a feel for cues. Sometimes it's easier to get the creative juices flowing (no pun intended) by setting the scene from the outset, and test the waters to see if they like the particular role I'm playing. Other times, and when my partner doesn't seem to be dropping any clear hints about their preferences, I like to ask about their fantasies; what's on their mind; what's on my mind; if they like what they see (because I like what I see), and for them to describe what they're imagining. Much of the fun in role play is spontaneity and reciprocation
My rule of thumb is to be safe, and persuade by suggestion than dissuade by force. I like to disclose that I love a good role play, and I want my partner to get as much pleasure out of it as I do. There are as many different boundaries and thresholds as there are people, after all.
Apart from that, just get a feel for cues. Sometimes it's easier to get the creative juices flowing (no pun intended) by setting the scene from the outset, and test the waters to see if they like the particular role I'm playing. Other times, and when my partner doesn't seem to be dropping any clear hints about their preferences, I like to ask about their fantasies; what's on their mind; what's on my mind; if they like what they see (because I like what I see), and for them to describe what they're imagining. Much of the fun in role play is spontaneity and reciprocation
"Each night has one sound I know: the moon against the water like your cheek across mine in another life." – Sara Eliza Johnson