12 minutes of suspense are far better than 12 seconds of surprise. It's really cool to surprise the players and all, but surprise is less useful and much weaker (story wise of course) than suspense). Hitchcock gave an example once which went something like that:
2 people are speaking over a dinner table in a restaurant when suddenly a bomb that lied underneath it blew up and the characters have been hit. We've got here 4, maybe 5 seconds of surprise. Now, imagine that the camera showed the ticking bomb from beforehand. Now, every line in the dialogue, every beat of action, is full of suspense. When the bomb finally blows up, there's no surprise, but we've got 4 or 5 minutes of suspense.
That's the whole thing. If you want to surprise the characters, think about the way you want to do this. If you can find a way to make the same thing, but with replacing the surprise with suspense, it's far better. Think about this situation: The characters arrive back from their mission and are on their way to the mayor's house to get their reward. When they arrive there, they find his body. We strived here for surprise and we got it.
On the other hand, wouldn't it be better if we would have plant clues in the scenery that something bad is gonna happen? The sun is bloody and red, the town is silent, and people go from place to place wearing black... Now, the players start to feel this tension, this suspense that something bad has happened. Suddenly, we get far more powerful time with this mayor's death.