If you need to draw some lines that can help you to draw the final image properly, for example a geometrical construction or a perspective grid, you can use styles available in the sketches page of the palette.

Starting sketch.	Cleaned up drawing over sketch.
Drawing with hidden sketch.	Final drawing.

The advantage of using a sketch style is that as soon as you want to hide them or you don’t need them anymore, you can switch them off in the final animation. Or you can easily select all the strokes made with a sketch style, and delete them.

Strokes done with sketches styles don’t interfere with fill operations. In this way you can start filling regions with other styles even if you’re still creating the drawing and you still need sketches as reference.