The expression ‘adding detail’ or ‘adding details’ to me implies that the existing drawing already resembles something that needs further detail. If the initial rectangle to you resembles a flag or a house, then you can add ‘detail’ or ‘details’ to make it complete or to make it recognizable. If the initial rectangle is just a box, it needs no detail to continue being a box.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.