It’s certainly very improv heavy, although having a comprehensive list of threats does help with that by giving you ideas so you’re not making things up in a vacuum.
As for stopping the PCs from going too far, that is a tricky problem. The first time I ran Apocalypse World I had one PC kill another in session 2, and that player then never came back (to be fair, she has some other problems with other players around the table). The solution is threefold:
- Talk with the players before you play. Set expectations. Tell them “drama and conflict is cool, but it’s cooler if you can make it last. Be as cruel as you want to each other, but make it so the player wants to come back and settle the grudge”
- Another mechanic that I borrow from Apocalypse World; make the PCs really hard to kill. If someone finishing blows a PC, then they’re dead as far as the one inflicting the blow is concerned. Have the “dead” character left behind in the dirt, maybe even bury them. Then have them show up again a few days later with brand new scars and one heck of a grudge, maybe represented by a new flaw. If every PC gets one of these then it makes other PCs think twice about “killing” someone.
- Occasionally, just let it happen. As long as both players are fine with it, there’s no reason to stop it from happening. Real example from the game above: Vega ordered Stomp into a field of carnivorous sheep to gather the wool she needed for her settlement’s winter clothes. Stomp reluctantly complied after some heavy threats, and then died in the process. I asked Stomp’s player how he wanted to do his “come back with a vengeance” thing, and he took one look at Vega (who was coming to the realisation of what she’d done) and went “Nah, I’ll let her suffer.” He rolled up a new character, and Stomp went down in history in a way that was satisfying to the player (until someone brought him back, but that’s another story)