I've been doing this for a long time. I know what I want to cover and what I consider inessential. But students have ideas about what they want to know, what they need to work on, what interests them. You won't find out unless you ask. Then you can let them design role plays that deal with issues that matter to them or that employ techniques they have encountered in real life.
Meaningful problems are better teachers than teachers.
You can't teach students to listen unless you listen. Use the Least Intrusive Intervention. The whole idea of negotiation role plays is that they provide students an opportunity to experiment in a situation where there aren't real dollars riding on the outcomes. One way to note things for the rest of the class without interrupting the players in the fishbowl is to use a flip chart that the players can't see. If it is necessary to intervene while a role play in going on there are several ways to do it.