Heya dudes and dudettes. I’ve come across a fast and suggestive technique for fleshing out sketches. I got the idea from a tutorial posted here which mentioned using colour palettes. Get a photo with a colour and texture that fits the sketch. Use the clone stamp tool on low opacity to drop elements/ texture/colour from the photo onto the sketch. I used a photo of rust to flesh out a sketch of a batman knight. If you’re lucky this can be surprisingly suggestive and can add bump from the photo to the picture- try adding interesting elements from the photo to the eye area of a character for example. It has the benefit of a palette with coherent colours.
Add light and shadow and Bob's your uncle. Unless you feel like taking it further, which as you can see i didn't.