Your description can do so much more
Description in a novel is more than just explaining to your reader where your characters are.
Yes, it’s incredibly important to do so. No reader likes to be in a metaphorical black box listening to people speak. This is where they are if you don’t give any sort of explanation as to where the characters are. If you say they’re in a living room, well, that could be just about anything anywhere. You really need to give some sort of description, but then the really tricky part comes into play.
What do you describe and how much and when?
If you give a paragraph-long explanation of that living room, your story will stop dead while you do so and your reader will get bored. They don’t care about the hero’s great Aunt Fanny’s knick-knacks that his mother has carefully placed on the bookshelf. But maybe those little ceramics have a purpose so you know that it’s important that the reader know about them.
If that’s the case, go right ahead and tell us about them, possibly even in painstaking detail, but if you do, you’d better use that information in the not-too-distant future.
Everything you describe should give some sort of information either about the person to whom these things belong or about the person noticing them. (I don’t think I need to point out that what is not noticed may also be very important. For example, while the hero may be noticing all the knick-knacks because his mother had a set exactly like them, he may not notice the million-dollar painting on the wall which might be an essential clue as to who the thief is.)
There is nothing better than writing doing multiple things at once. Yes, detailing the setting is important so the reader knows where they are. But remembering whose eyes you are seeing that setting is equally important. Your description will tell us so much about that person—both what they notice and what they don’t. The description could also inform the story, as in the example in the previous paragraph.
So, when do you write this incredibly important description? Do you write it as you’re writing the book? Do you write it later layering in the meanings, the red herrings, the underlying history of the character through whose eyes we are seeing this setting?
This is, of course, up to you. I would recommend doing both. As you are writing, put in that description so that you know what the setting looks like. If you are writing from deep within your character’s mind, you will probably unconsciously describe only those things that are important to the character. Later when you go back and reread your description, that’s when you can think about how much is being shown. If your descriptions are not doing anything more than describing the setting, then maybe you want to consider whether it can do more. Whether it needs to do more.
Sometimes a setting is merely a setting. But even then, don’t forget to put it in and have it be, specifically from one character’s point of view. And please, please, please, no info dumps!