How is a big, dramatic, meaningful scene determined? Part of the fun of an unscripted game is that even the DM doesn't know what encounters might be any or all of the above until it plays out.
As a DM I don't "plan" haggling scenes with merchants or casual conversations with city guards. The players initiate these things which may become important or just a minor aside interaction depending on what happens.
I don't "fast forward" the game toward certain scenes and situations. The players can certainly do so if they wish since they largely control the pace of things.
Sorcerer, Dogs, Burning Wheel, and The Shadow of Yesterday are all pretty similar - they rely on some pre-game focus being set (Kickers and the definition of Humanity in Sorcerer, the Town Creation in Dogs, Beliefs in Burning Wheel, and Keys in TSoY), usually by the players; this provides the group and the DM with an idea of what the players want to focus on. When you play you focus on those things and those are your dramatic scenes.
You wouldn't spend time chatting with city guards in Sorcerer unless it had something to do with your Kicker and/or Humanity.
These games are unscripted - they would not work if they were. If someone (anyone, either player or DM) determines how a PC's Kicker/Belief/Key is resolved (or how the trouble in town is resolved) before play, when the point of the game is to see how it gets resolved and changes the character through that, why play through it?
I was playtesting my 4E hack tonight and I approached the situation totally differently - because we've got a different agenda with these games. I told the player where the PCs were, what they saw and heard, and let the player decide what to do with that information. We ended up with a lot of minor RP - talking to farmers to get the general situation, chatting with an apprentice blacksmith, buying herbal rat repellent from the hippie herbalist.
The PCs would do something, I'd describe what happened; if we needed to we'd roll the dice. Once the PC's action is resolved, I'd ask the player what the PCs were going to do from there on. I let the player set the pace of the game.
In this game all that matters because there are important decisions to be made and important information to gather. The player has to be the one to set the pace because the pace he sets is another important choice. If he had decided to keep watch on the town at night instead of just going to bed he would have seen someone slip out of town.