Every Whosprite begins with a quick sketch, done in ballpoint pen.
Like these ones, of 11th Doctor Matt Smith.



I put a photo or screen cap up on my screen, with a grid on it dividing it into 4x4 segments. I take a piece of Xerox paper, fold it into 4x4 segments, and draw. The segments are helpful in keeping track of the proportions of the character, although the proportions will be wrong at this stage anyway, as I'm just sketching.
I draw quickly and usually draw several versions of the same character in a row.
These rough sketches help me get my bad ideas out of the way, so I can see which photos will make good final color pieces, and which won't, for whatever reason. The best of the sketches are selected to be redrawn and inked. They will also have their proportions carefully corrected using the photo as reference - an important step in achieving the necessary realism. I used to do this last, but lately I've been doing it right after the sketch is done, so that the inked version has the correct proportions.
The sketches present my work at its most raw and unrefined. Still, it's clear to see which ones are best and will make the best final pieces.