Here's a look at some of my process when it comes to creating crowd scenes or in this case a puzzle for ChickaDEE.
First I start off with a basic layout based on the client's requirements for the puzzle. It's super rough just to find composition and perspective. I do a bunch of these little thumbnails. Once I find one I like I scan it in and blow it up to full size and then print it out. Of top of that I'll start roughing in bodies of characters, and refine them up a bit more with a layer of tracing paper on top. Once they've been placed and their general actions decided on I'll then lay another layer of tracing paper over top and then refine the drawing more.
Now everything should be looking good! The next step is scanning and taking the whole thing into Photoshop. Once scanned I'll make a few adjustments I can see are needed then send the rough to the client for feedback. I'll make their adjustments as required then it's on to colour!
Usually I have an idea of where I want to take the colors but I always do thumbnails just to see how it will work. Sometimes, depending on the project, I'll add a step in between the sketch and colour, which is a black and white comp to figure out lighting. In photoshop I'll do a painting under the drawing, and flip on and off the line work to see how my colours are working. In the animation it only shows one color rough, but it takes at least a few (sometimes more) little paintings to figure out what will look good for the project. I'll keep this rough up in a separate window when I do the final illustration for reference.
With the sketches done and the colour figured out it's onto finals! I flat out all my colours in Photoshop then start refining them up and shading them with different brushes in Photoshop. I usually keep a desaturation layer on top so I can be sure things are reading okay in tone as well as in colour. Sometimes I'll make minor adjustments to the colour along the way if I find it's not working as I thought it would. After some hours of painting the final illustration is complete.
Hope that made sense!