One of the best things about Photoshop (and there really are quite a few) is the way it sits just as comfortably in the hands of the digital artist as it does in those of the photographer or graphic designer. What you have is total versatility from one extreme to the other: from complete handling of "pure" photography to more "painterly" illustration and graphic possibilities. But it's the creative free-for-all that exists between these two extremes that is most exciting for any digital artist.
The digital illustration process is almost always made up of three stages. Following the first stage - the initial brain-to-paper stage - the next thing to do is to create a version in Photoshop. This stage is concerned only with putting all the necessary elements in place and getting the composition just right. The odd rough edge at this point isn't critical, and neither are the colours just yet.
The third and final stage is the "colouring-in" stage - the bit where you draw and paint over the composition you've laid out. And it's here that you can start getting stylised, to work on bringing out the best aspects of your illustration. After all, a good idea is one thing, but how well you carry that idea off through your work is quite another.
In this tutorial, you'll be taking over an illustration at this third and final stage. With all the elements in place, you'll be looking solely at adding depth and atmosphere to the scene with some dynamic lighting effects. First, you'll focus on enhancing the light and tone in one specific area of the picture, a technique you'll then be able to apply to the rest of the scene, before casting a few shadows to create that extra sense of atmosphere.