I think that the best way to improvise is to prepare far in advance.
It might not make sense at first, but in the game I am running, the PCs are working for a dude operating under both the names "Thermidor" and "Raphael" who is working for Damon (they don't know this last part) to essentially start a mob war that will weaken the syndicates in Boston enough that Damon can swoop in and take total control of Boston's underworld. They heard Damon's name dropped and were looking into him for totally different reasons (they bought into the corporate line, at first, that Damon was the dragon who had attacked NeoNet Towers and crashed into Fenway). They decide to go check out one of his clubs. I had not been expecting this, and it has nothing to do with tonight's run. However, I have enough moving pieces to the larger plot that I decide I can drop some teasers.
I have my party role edge to see how much they can find out here, and they get one hit with a glitch. Alright, so they learn something, but the manner in which they learn it is not enough for them. So far, they've only done a single job for Raphael (which sparked tensions between the two mafia families), have no idea he's connected to Damon, and don't know what the big plan is. They don't know this either, but I decide that he's meeting with the Vory right now to try to con them into getting involved in this mob war. They aren't there five minutes before a bouncer places his hand on the decker's shoulder and tells him "Raphael doesn't know how you found out about his meeting, but he says you need to leave. Now." Just before they're ejected from the club, the party makes good on a perception check and sees a couple of dudes with weird tattoos guarding the door to a back room. Definitely not club security. They run the tattoos through a matrix search later and learn they are the mark of a thief-in-law (Vory grunts)
So with roleplay and research, this detour ate up almost half an extra hour of game time, but because I've plotted out what's happening behind the scenes, I've been able to show that party that Raphael is meeting in Damon's clubs, that he's talking to the Vory, and now he also doesn't much care for them because he assumes they're trying to snoop on him. They actually may end up fighting him later, because the decker HATES him. Then they get to learn the hard way he used to be a Seraphim!
