Feathers only rarely fossilize, so the distribution of feathers in dinosaurs is difficult to determine. Sometimes feathers mark the bones, though, and bones do preserve well. Here’s an example: the forearm of a Velociraptor retains an array of small bony bumps evenly spaced along its length. What could they be?
In the photo below is the homologous bone of a turkey vulture, showing similar bumps. They are quill knobs, or places where a ligament anchors the root of a secondary feather directly to the bone. Their presence is an indicator of the presence of a large feather — something more than a little fluffy down — at that location.
The top panel of that image is the Velociraptor bone, and some rather more subtle bumps are there, spaced about 4mm apart, suggesting that there were about 14 large feathers hanging from each arm of the dinosaur.
Velociraptors didn’t fly (although if they did, Spielberg would have had real fun with his dinosaur movies), so the unanswered question is what they did with the feathers. Shielding nests? Generating negative lift like a spoiler during running? Knowing what animals do, I’m inclined to suspect they were used for sexual displays … which would imply that they could also have been brightly colored. The special effects people should be more imaginative!
Turner AH, Makovicky PJ, Norell MA (2007) Feather Quill Knobs in the Dinosaur Velociraptor. Science 317(5845):1721.