What are the best ways to create characters that interact with their environment?

They should be inspired from reality and your own experiences. Without that the audience will not be able to identify with them.

For 2d animation, an important thing to be said here is, in the visual conceptualization stage of the project, there must be a great study on color, shape and contrasts of the character in relation to the backgrounds, to prevent it from disappearing into the backgrounds. Isso acontece geralmente quando o personagem na possui outline ou quando o backgrounds e o personagem possuem outlines da mesma cor. This is why this study of contrasts, color and shape is important in the conceptualization stage and in production. The solution is often to put a black or dark outline on the character, Leaving the BG without an outline.

"Enhancing believability in animation" Animation can span many styles and genres. It can be hyper-realistic or fantastical, abstract or concrete, cartoony or indistinguishable from real life. But unless an audience believes your animation, none of the above matters. Characters interacting with their environments is a good first step. Stand your character on the ground, maybe cast a shadow. Hold and use props, sit in chairs, eat food, wear clothing, open doors. Anything outside of ourselves that we are accustomed to interacting with and seeing in our daily lives represents an opportunity to add more believability to your animation. Create a fulsome world for your characters to live in, and the rest will take care of itself.

Movie producers often repeat, "Music is 50% of your film." For animators, sound effects are 50% of your animation. Sound is physical (interactions with the character), environmental (a situation the character is experiencing), and musical (a thematic feeling). Sound can punctuate movements and reactions, carry scenes, or set a mood. Sound effects often come from sound libraries (SFX). Many examples exist, from free resources on the internet to well-curated collections requiring a license. You can also create sound effects using either field recording with a microphone and adding the effects to your animation later or using Foley by watching the animation and performing the sound effects in real time while recording the result.

Es importante tener en cuenta la naturalidad de los movimientos. Muchas veces se cae en el error de exagerar los movimientos, afectar la velocidad o ser demasiado específicos en alguna parte de la anatomía del personaje. Esto solo provoca que el movimiento no sea orgánico y se vea robotizado o antinatural. Es super importante entender la personalidad de tu personaje y su entorno. Pero todo se unifica cuando entendemos la intención de nuestro personaje para hacer cada cosa.