In football, sometimes a quarterback's body language will reveal where he intends to throw the ball long before he makes the pass. They call this telegraphing the pass, and smart linebackers can use the quarterback's telegraph to, well, crush some guys.
Similar things happen in scenes all the time.
Sally is folding her laundry and musing over her lost love from college. If only he hadn't graduated and then rushed off to Marzipanistan to infiltrate that rogue terrorist cell! And he'd never been heard from again. Sniffle. And today would have been the tenth anniversary of their first date. If only he had lived. Her one twoo wuv. She could be folding his socks right now. Sniffle.
Doorbell rings. Guess who's back from Marzipanistan? College boy! Shock!
Sally is out for drinks with her posse. Appletinis for everyone! Soon, talk turns to how awful their mothers can be. Sally excuses her own mother's occasionally awful behavior by explaining how insecure Mommy is because of childhood events, and how she learned to cope by frequent spa visits and obsessive grooming.
Sally stops by Mommy's house on her way home from the bar. Mommy lays her out for being windblown. Also, how dare Sally wear that frumpy dark blouse after Mommy spent ninety-seven hours in labor with her? Shocking.
Do you see how this works? And it's not very effective, is it. The writer is telegraphing conflict before it erupts, and smart readers everywhere are likely to go linebacker as a result. Why eliminate the dramatic surprise by undercutting it in advance? Writers sometimes say, "But the reader won't understand why this is a surprise/problem/whatever if we don't explain the backstory before the event arrives."
Except, guess what? People like a surprise. And if certain information is necessary to allow the reader to interpret the character's reaction, then that information is best placed in the context of that reaction. Not elsewhere. Readers are smart enough to follow along when a few extra details are braided into the scene, such as:
Sally opened the door and saw a ghost standing on her porch. No, not a ghost. Vincent.
"But you're dead," she said, and then slammed the door closed before he could respond.
Oh, God, oh, God, oh, God. The only man she'd ever really loved -- how was this possible? They'd sent her his wallet, his bloodstained shoes, a sympathy card from the wife of the resistance leader. She'd spent years trying to recover his body from the dictators in charge of Marzipanistan.
Maybe it wasn't him. Maybe it was some guy who just looked like him. Maybe she was imagining things. She was just lonely, that was all, and had been reminiscing about the good times with Vincent because this week would have been the tenth anniversary of their first date.
The man -- not Vincent, it wasn't possible, it was probably just the paperboy -- tapped softly on the door. Tentative. "Sally? Open the door, Sally."
But instead, she leaned her forehead against the cool, sturdy wood, and fought to keep the walls from spinning around her.
By building the right details into the scene as it unfolds, you can safely eliminate any pre-scene set-up and let the scene itself carry the weight of the drama. Use the "set-up" information to build detail into the character reactions -- if that is the information the reader needs to understand the reaction, then that is where it should be contained.
This might seem like a fairly obvious technique, but honestly, we see telegraphed surprises all the time, usually in the early chapters where authors might feel more driven to set up their story mechanics. But really, stop worrying about setting things up. Trust the scenes. Trust the conflicts. And most of all, trust the reader to be able to follow along.