First, I'd recommend checking out a couple older Tips & Tricks articles from back in the day related to acting:
The quick answer would be that as an animator, your job is to bring the character to life, which means that in a very real way, you have to be an actor. Your job is to BECOME the character, especially in your reference and planning, if you want the performance to be unique and believable. This is a must.
As animators, we don't necessarily have to know *everything* about acting. Generally, we are being given an existing line reading, for example, but we also must know how to dissect that line reading and figure out what the actor was thinking when he delivered that bit of dialogue. What was his subtext? What is the operative word? What are his motivations? What does he WANT most in the scene, and what is preventing him from having it? Sometimes you have to make some of this stuff up to create a whole picture that you can work with, but it's very important to go through the process.
To me, the main stuff I find that I apply to animation is: subtext (it's often better to animate what the character MEANS rather than what they are SAYING), operative words, backstory (who the character is, where they came from before the scene, where they will be after the scene), the desires of the character, and the obstacles keeping them away from their desires. In my video reference (using all of the above), I always try to truly get into my character's head, and TRULY feel the emotions and desires the character is feeling. When I don't quite get it to that level, my reference always ends up being useless and predictable. However, when I *do* try hard to become the character as much as I can, the reference is always a huge help.