As an NYPD negotiator I had absolutely no problem remembering that I wasn’t the commander: He was generally the guy in the room with the shiniest badge and the largest hat size. In most everyday situations, you can keep the roles separate by planning for them to be separate before negotiations begin.
Plan to negotiate, plan to break when you’ve got your best possible deal—when you’ve nailed down all of the difficult issues. Plan it right down to the exact words you want to use. If you’re negotiating on the telephone—a favorite in hostage negotiations, since it’s a heck of a lot safer than standing ten feet away from a guy with a gun—write it at the top of your notepad. Hey, tattoo it to your wrist if you have to. There are worse things on people’s arms, believe me.