I'm a new GM (the system is DND 4.0 but that's not relevant here). I've quickly found that I perform very poorly when I'm portraying a negotiation between the PCs and one or more NPCs. Even when they have common ground and could easily find something to agree on that would benefit both sides, the negotiations end up with no agreement reached and both parties annoyed at each other. (Of course, when they're "negotiating" with the BBEG, this is the desired result, so this is really only about friendly or potentially friendly NPCs.)
After a recent negotiation failed, I talked to the players out-of-character and said "This is where I wanted to be going with this" and they very graciously hopped back on the railroad, so it's not at risk of derailing the adventure, but how can I learn to portray the negotiation in a realistic and meaningful manner that's compatible with how I want to move the story forward?
To expand: the recent negotiation was between the town sage, who wanted to know everything about the PCs (the meta-reason for this is I wanted them to develop their backstories further, so I can tailor the adventure to them). The PCs wanted the sage to examine an artifact they found and give them some history of the area, hoping they can get him to reveal location that might be the BBEG's headquarters. The result was the PCs got suspicious of the sage interrogating them, and resented his air of superiority, and utterly refused to divulge any details. At that point I explained what I was trying to do; two of the PCs consented to stick around and answer questions.