Details are one of the things writers can use effectively to create a living world in the readers’ minds. When describing the details of a location or an object, for example, details can serve to highlight an element of the reality. Let’s imagine this description comes from a Steampunk fantasy story:
Cyndee peered at the lock. The intricate latticework around the old-fashioned mechanism had been carefully fashioned from gold wire in the shape of vines and leaves.
In the above case, the writer is trying to elicit the idea of an ornate world. In our world, a lock is a lock. The attention to hand-crafted things is one important part of Steampunk. Moreover, it’s clear that the lock is important to the plot (we can say, for example, that Cyndee has been looking for the place to use the ornate brass key she was given a few chapters earlier), so using detail to enhance the moment is an effective use of detail.
Conversely, details, when misused or placed on the wrong element of a scene, places importance on things that the reader doesn’t need to know. Let’s say the following comes from a gritty urban fantasy:
The flat-screen television monitor, fifty or maybe fifty two inches wide, mounted to the flat grey ceiling by six large bolts aligned in two rows front and back, with a large black cable coming out of the back that disappeared into the ceiling, showed the destruction of the city Keylan had left just two days previously.
Although there is some merit to creating the image of a dingy reality where big TV screens hang from ceilings everywhere, broadcasting propaganda to the citizens, the writer needs to consider where and when to place those details. In this case, we can argue that Keylan’s focus would be on anything but the appearance and location of the TV monitor (and in fact would most likely be on the destruction of the city).
If there was a rule about exactly when a writer needs to world-build, writing would be a lot easier. But let’s say that as a general rule, introduce things like a dingy room with a grey ceiling and a TV monitor earlier in the story, as the character first meets them. Keylan’s first impression of the building, or room, should convey the details needed. When the TV begins to show the destruction of the city, however, the writer should instead focus on what’s being televised.
If your characters are walking toward a trap they’ve been warned about, spending a lot of time explaining how the characters shirts look is a waste of detail. If your main character has just suffered the loss of his best friend, explaining that there are seven chairs in the room, all set up against the far wall, which has a window facing south-southeast, means you’re not spending time on that character’s emotional state. If the two characters in your romance are finally alone after a crazy day in which his mother demanded her mother not attend the wedding, and the dog ate all the wedding invitations, explaining that the sofa they were sitting on was made of imitation leather in a shade of red-brown that didn’t match the powder-blue pillows or the purple comforter sitting on top of the sofa’s back, is not the best use of word-time.
Always make sure there’s a reason for providing details, and make sure the details are in the right place.