For Ratatouille, Pixar's own team asked: "How can rats win empathy? It's difficult." When Brad Bird took over the film, its rats "looked more like humans". The easy fix goes further: scientists call it the baby schema, big head, round face, big eyes, and it makes faces cuter, even dogs and cats. Bird did the opposite. "At some expense", he had the rats rebuilt on all fours with their muscles working correctly, after his team "observed a couple of real rats for a year". Then Pixar kept the rat and hid only what repels: "by cradling the teeth within the lips they appear smaller and more appealing", an exposed gum line makes a character less appealing, and "Simple shapes also help the rats feel smaller." Standing became Remy's choice, "more or less ratty" with his mood. And in 1987 John Lasseter warned that appeal is "often misrepresented to suggest cuddly bunnies and soft kittens": it's whatever holds your eye. Bird's rule: "If you understand the real behavior of rats, you can deviate from it with knowledge."

The rat here is our own drawing, not a film character.

Which movie trick should we take apart next?

Sources: Sonoko Konishi and Michael Venturini, "Articulating The Appeal", Pixar Technical Memo 07-12, SIGGRAPH 2007; Brad Bird to Paul Fischer, Dark Horizons, June 2007; Brad Bird to Bill Desowitz, Animation World Network, April and June 2007; John Lasseter, "Principles of Traditional Animation Applied to 3D Computer Animation", SIGGRAPH 1987; Glocker et al., Ethology 2009; Borgi et al., Frontiers in Psychology 2014.
Sounds: our own, made in code (earshot, CC0).