Just another short post to illustrate the effectiveness of using contrast.
This is a story sketch from Dumbo I scanned from the Disney Archives book "Story".
There are a couple of contrasts in the drawing that elevate it from just a serviceable story sketch to being an effective picture all on its own. The gesture line of the elephant is a strong direct line that gives the push against the cage a lot of force. The gesture of the tiger pushing back against the direction of the elephant's gesture gives more power to the elephant's push by contrast. It's like when you paint two contrasting colors next to each other - they both become more vibrant through contrast.
Also the contrast between the two animal's attitudes works well to strengthen the picture. The intense strain on the elephant's face is made much stronger by the contrast of the lazy, satisfied-looking expression on the tiger's face. Also, it seems to me that the tiger's paw, the wheel and the elephant's knee do a good job of creating a good composition and keeping your eye centered around that area of their expressions.
I redrew the picture without the tiger to show how much is lost when the tiger is removed. The force of the elephant is diminished and the circus cart looks a lot less heavy and easier to push (the original is right below it for comparison).
Also, the composition is seriously hurt by the absence of the tiger. Without the tiger there, there's no element to keep your eye from wandering aimlessly around on the right side of the frame. The tiger helped to make it a "closed circuit" with a clear circular path for your eye to follow.
The purpose of any story sketch is to tell the story clearly, and an individual sketch doesn't need to be a great piece of art - it only needs to work with all the other sketches to tell the story in the best way possible. But I like how the addition of a little contrast helps make this a great picture, even when seen all by itself.