For those of you who have purchased pre-written adventures (rather than writing your own), you are familiar with boxed text. It is text that is usually set aside from the rest of the adventure and intended for you to read aloud to your players. "The Jaded City of Oz", the sample adventure in my book, includes such "boxed text", though I mention it in quotes because it's not boxed.
The idea is that the guy who wrote the adventure is helping out the Narrator by feeding him descriptions of the various things that the player's characters encounter. This is good. But there is a wrong way to do it. Some friends of mine are currently playing through a pre-written adventure for D&D and laughing at a lot of the mistakes that the writer made in writing these descriptions.
Overwrought: The "dungeon" in the scenario is actually a haunted house, so the writer is clearly going for "spooky" descriptions. But they can easily sound clumsy, like the writer is trying too hard to be scary. And they tend to be long, comparatively speaking. Especially since each room of the mansion is accompanied by a paragraph of spooky description. This really undermined any attempt at a horror scenario, as we found ourselves making jokes about "save vs. boxed text" (for non-D&D nerds, a save, or saving throw, is a die roll made to avoid a particular hazard).
Useless: In at least one instance, a paragraph of supposedly evocative boxed text is followed by "There's nothing of interest in this room." Meaning that the full minute we spent listening to the Narrator read that paragraph was pretty much wasted. Good boxed text should present the players at least one thing in the room to interact with. It doesn't have to be plot relevant, but it should be something. Give me a brick wall so I can search for the secret brick that opens the hidden door. It doesn't matter if the hidden door doesn't exist. It would have been more effective if the Narrator had simply said "The room is creepy looking. Here's the door to the next room."
Inconsistent: While the main floor of the haunted house had long descriptions, by the time we got to exploring the basement, the descriptions got a lot more succinct. What happened? Did the writer get tired? Run out of synonyms for "dark"? Even though there was a very scary monster in the basement, the boxed text provides no build-up to this.