Animators are usually provided with some reference of the actors reading their lines, which is extremely useful.
You can pull little facial nuances or body movements that can help make each character unique. Most of the time, we film our own reference or enlist the help of someone else in order to explore and refine ideas for our shots, the whole time trying to keep character traits in mind to make our reference as close to the character as possible.
When we're unable to film our own reference, for example, of a big cat running, there are so many resources out there. YouTube and BBC motion Gallery have been useful, but with the internet available to us, there are many resources out there, so there's no excuse to not have good reference.