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I took my daughter to see Kung Fu Panda, which is one of the first movies she has seen. It amazed me that after weeks after the movie she could clearly recognize and excitingly point out the characters from the movie that were on advertisements, tshirts, and even at McDonalds.
She is two years old and she sees lots of panda bear pictures and stuffed animals around but only when she sees Po from the movie, she gets excited and tells me it is from Kung Fu Panda. The same thing happens when she sees the tiger from the movie.
The point is this, your characters have to be unique. You can use accessories, clothes, unique facial expressions, over-obvious body features, and much more. They should also all be a similar look and feel to them. So go back and make any needed changes to your characters.
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