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Tom is just more detailed than the mice, because his drawing is so much bigger. The bigger your character is on screen, the more detail and contrasts in sizes you can take advantage of - if you know what you are doing.
Tom becomes simpler once you understand the basics of construction and facial mechanics .
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FACIAL MECHANICS BASIC:
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Eyes
Eyebrows
Mouth Shape
Cheeks
Those 4 basic parts can add up to an infinite amount of expressions. They each can be broken down into sub parts:
Eyes have pupils.
Eyebrows have accompanying brow wrinkles and sometime individual hairs.
The mouth has an upper lip, a lower lip, a hole when its open, teeth and a tongue.
All these sub forms can help define or refine the basic expression made by the basic parts.
For example:
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No lines on a face should ever just float around in space. Every line should have meaning and sense.
Knowing all this, you can then break down logically how this drawing of Tom's head, face and expression works, step by step.
Starting with the main features, then working down to the details.
And making sure the details obey the directions of the larger features.
WORK OUT BASIC CONSTRUCTION AND BASIC EXPRESSION FIRST
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THEN WRAP THE DETAILS AROUND THE BASIC FORM AND EXPRESSION
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But you gotta practice and copy good drawings using the construction method if you are to make any real gains in skill and understanding.
Just thinking about it won't make your drawings any better.