In How Would You Describe It? I was talking about this thing called Description, which seems to get so many beginner-writers worried, and how you can get better at it. But I don't myself have a mental category of writing called Description at all; I just think in terms of Dialogue, and Everything Else - or, more grown-uply, Narration.
That's not because evoking places and things isn't important. The places and spaces we live in, and the things we live with, are profoundly important - but notice that they're important because we live in and with them: it's how we interact with them that matters. You don't get a lot of description in fairy tales, or in Jane Austen: story is king. What furniture is in a room matters because who sits so you can't see their expression matters, and how full the room is at the wake does, and how close together the illicit lovers can stand does. What a landscape looks like doesn't matter: how it makes a character move or think or dread does.
Take, for example, this extract from The Mathematics of Love. That passage isn't just word-painting to "help the reader imagine it": it's using bits of Stephen's memories to embody how he's sorting through his present and moving into his past, as he works out which parts of the past he can bear to re-visit. Separately, it's a way for me to evoke for the reader Stephen's backstory which he, as a narrator, rarely refers to. And, thirdly, it's a way of preparing the ground in the reader's mind, in case Stephen does go back to Spain. So, even if the character's action is only thinking, settings and objects aren't separate from the workings of character-in-action, but part of them, and conditioned by them. My purpose was to move Stephen from emotional Point A to emotional Point B, from where he would then act: it just happened that, because I used memories to do that, it came out very descriptive.
It can also be useful to work with settings and objects when you're trying to cover the ground: November became December and the tweed on the loom grew steadily, while under her window the horses stood with their heads hanging in the rain. That's from my Showing and Telling post; it's essentially descriptive, but it's there to exploit the changes in these things, in order to evoke time passing in a direct, physical way. Mind you, it might double-up as the way we learn in the first place that the MC does weaving and owns horses, just as Stephen's passage doubles-up in colouring-in the reader's mental picture of Spain.
And talking of using description to prepare the ground, description is very useful for introducing apparently ordinary things which are going to be important to the plot, before you need them. I call it the Reverse-Chekov Tool, (not Rule because there are no rules). Chekov said that you should only have a gun on the wall in Act One if it's going to be fired in Act Three; Reverse-Chekov states that if you need a gun to be fired in Act Three, you should make sure it's on the wall in Act One. If Stephen goes back to Spain and the effect on him of "now" is partly because of what he remembers, one side of the contrast is ready and waiting.
In other words, Reverse-Chekov means you don't have to pause the Act Three car chase while you explain that post-war Citroëns had all-hydraulic suspension, and that the road runs down a rocky gorge topped with trees on either side, so a single bullet from an invisible gunman can put a car wholly out of commission (Gavin Lyall's Midnight Plus One, since you ask). Fiction is always working with the human drive to make patterns out of the random chaos of real life. For it to be simultaneously shocking, and natural, for the car to be shot in Act Three we want to feel, retrospectively, that the risk was part of the normal texture of real life in the story: neither entirely random, nor over-determined. So, how and when are you going to build in descriptions of each element - how Citroëns are built, the geography round Grasse, how professional gunmen work - for maximum drama?
Of course, an internal narrator makes it relatively easy to decide what gets described and when: of this setting or thing, what would this character-in-action know, notice, and think about it? It's harder to choose the right bits and right amount if you've got an external narrator, because that narrator could narrate anything, including half a travel-book's or antiques-guide's worth of Stuff. I think the key is to ask yourself questions such as:
1) Do we need any description of this place or person or thing? Or are you writing it partly just for the sheer writerly pleasure? Your reader may take pleasure too - especially if acute observation and gorgeous prose are among your strengths - but it's always at the risk of losing narrative drive.
2) Why do we need description of this place or person or thing? Plot? - because it matters to the action now, or later? Character? - e.g. her take on this place is revealing of her? (but careful: don't stall the action with an unconvincingly long series of observations). Character-in-action? - e.g. his take on this place brings about some internal or external change in him? Atmosphere? - can be wonderful, but needs handling with discretion because it's not action.
3) Why do we need it now? Would earlier, or later, or from a different point-of-view, be better? Or should you break it up and distribute it between the possible points in the story and points of view?
4) What's the minimum amount of description which will achieve what you want? Or do you - as with clues in a detective story - want to tuck the crucial element a little deeper into the texture of real life and all its red herrings?
5) Are there ways of making this description more active while keeping it vivid and evocative? (Leading us through the landscape? Evoking the box by describing how it breaks when it's dropped? Getting characters interacting and experiencing the menacing or idyllic garden/boat/house in a way that moves the story on.)
But of course, those all lead on - or, rather, lead back - to the real question, which is:
6) What am I trying to do with this description?
And if your answer is, "Because the reader needs to know what it looks like", then I'll spin you round and send you back to Question One. The reader doesn't need to know what everything looks like: sometimes, as I was exploring in Less, More and Apollo in his Chariot, the best and most vivid and evocative description is the one the reader makes in their own head.
Of course, you may want to write a full description of something because you need to know what it looks like - or you may do so "by mistake", as it were. If I'm putting too much Stuff in, then it's because I'm feeling my way, either in the early stages of a piece, or just because I haven't, really properly, decided what this scene is for, and so my character is wandering through the landscape while I find out. That's process writing and, as with all sorts of things which have a perfect right to turn up a first draft, you can, and you should, get rid of it later.
Once you've got used to thinking in terms of what the storytelling purpose of describing something is, whether you find Stuff falls out of your pen like rain over last Christmas, or your scenes are so bald that everyone says you should be writing for radio, you should find it easier to work out where you need description, and what description you need.