It’s not always a problem. “Luke, I am your father.” That’s got enough of an emotional punch that Vader can just say it – though notice how the fight scene had to build up to Luke’s physical defeat before this truly terrible news could be imparted to maximum effect.
Pure plot info, though, that can be a real pain. It doesn’t carry any emotional charge to connect it to the characters; it’s just some guff about what time the bank guards complete their rounds, or what will happen if the control codes aren’t entered into the starship’s computer. Stick it in as a plain statement of facts and it’s like shoving a stick into the spokes of a bicycle wheel. Instant momentum killer. Suspension of disbelief goes sailing over the handlebars and ends up nursing a bruise from which your narrative may never recover.
Television writers learn quickly how to deal with pure plot exposition. There was a good example in the first season of Desperate Housewives. One of the four leads has some major piece of local gossip that she needs to impart to the others. But, when she tracks them down they're in the middle of a coffee morning with a bunch of other neighbours who she can't let in on the secret. So she gets them out into the kitchen on some pretext and closes the door. Now she’s imparting this info to her friends (and us, the viewers) but she’s having to whisper – and she’s got to say it all fast before the others come back in. The tension, though completely contrived, distracts us enough that the exposition slips by quite entertainingly. We hardly notice we’ve just been fed a wodge of facts.
The most overtly expositional scene I’ve had to write for Mirabilis comes in chapter three ("Standing on the Shoulders of Giants") when Inspector Simeon tells us about the murders that have brought him aboard the train. The information is essential to the whodunit subplot that will embroil Estelle in more than a little trouble later. At the same time, it’s just a bunch of facts that the reader has no reason to care about right now.
So let's look at everything that actually goes on in that scene (see below.) Jack is riding high after saving the day with Stephenson’s Rocket. He’s cornered by Simeon and lets himself be cross-examined so as to cover for Gus, who is sneaking onto the train behind the inspector's back. Simeon gasses on but Jack is distracted; we know he's got other things on his mind. They’re in a narrow corridor and other passengers are squeezing past – specifically Sir Archibald Whitmead, who thereby gets a perfect opportunity to overhear what Simeon is saying.
Jack shows no more interest in Simeon’s theories than we readers have any reason to. But when he walks away, we stay with Simeon and Caitou. And now we learn that Jack’s nonchalance has aroused Simeon’s suspicions. (That’s another piece of exposition, covered this time by our amusement at Simeon’s sense of self-importance.) Ah, so it turns out that the plot details that Jack (and we) barely listened to could land him in hot water. See, now we’ve been given a reason to care.