To me, every scene in a book has to have a purpose: drive the plot forward, set up important information for later (the gun on the mantlepiece in the first act, ready to be used in the last act), develop character, or, ideally, all three.
Therefore, during the editing stage (absolutely no need to think about this kind of thing in the writing stage), you should review every scene with this question in mind: what purpose does this scene serve? Better that you ask it than the reader.
Since I’m still in the writing stage, I’m not going to use an example from my own work this week. I’m going to pick on a book I happen to be currently reading, Deadhouse Gates by Steven Erikson.
There’s a scene which consists of a conversation between two characters, discussing an argument that has just occurred off-page; the aim of the scene is to give the reader the upshot of the argument and also re-inforce some other information which has already been given. There’s maybe a snippet or two of new information, but not a lot.
All writers know how awkward it is to try to give the reader information the characters already know, especially in dialogue – it leads to the dreaded ‘as you know’ clunky information dump: which is all this scene is. If the argument was so important to the plot, why wasn’t it shown directly? If it wasn’t important, why waste time and the reader’s patience by sumarising it in dialogue? Couldn’t the snippets of new information have been woven into the very next scene?
The second question to ask is, is there a better way to achieve the desired purpose? Through action, not dialogue? With different action? Through a different character? At a different time in the plot? By increasing or slowing the pace? How does this scene contribute to the overall structure of the book?
It’s important, when editing, to remember that the scene as a whole should be considered, as well as the writing within the scene.
Next week, I shall continue to pick on Erikson with some comments about focus.

[...] Deadhouse Gates (a book in the Malazan Book of the Fallen series; last week I picked on one of the scenes). It’s epic fantasy: therefore almost by definition it has an awful lot of characters and an [...]