We know how expressive the human voice is. Meaning doesn't just come from the words, it also comes from how the words are spoken. Live actors use their own voices when playing a role and for many actors, whether dramatic or comic, the voice is a major tool for communicating with the audience.
Animators rarely provide the voices for the characters they animate. Instead, they're handed an existing voice track. Usually the animator is not present at the recording session and has no input into how the lines are spoken. The voice track is a fait accompli that the animator must deal with in creating the visual performance. As animator Tissa David once said, the performance is in the soundtrack; it's the animator's job to pull it out.
So far as acting is concerned, the voice track dictates several things to the animator. The first is emotion. A line of dialogue can be read different ways depending on the emotional state of the character. The actor makes this decision and the animator is tied to it. If the character is angry or hurt or confused while saying a line, the animator is forced to portray that emotion or the performance won't be believable.
Another thing the actor dictates is timing. How quickly or slowly does a character say a line? Besides having a direct impact on the speed of the mouth and face, the tempo of the dialogue may also dictate what kinds of gestures will work in the allotted time.
Finally, the actor determines emphasis. For the line "I'm not going to school," you can put the emphasis on "I'm" or "not" or "school." The animator's choice of poses will be different depending on which word the voice actor emphasizes.
The voice actor has to dominate the animator if the visual performance is going to work. In fact, the voice track is one of the things that holds an animated performance together, given that multiple animators will most likely perform the same character.
These things are true for any voice; they don't begin to account for the associations that a well-known voice brings to a character. Actors such as Robin Williams or Eddie Murphy remind us of previous appearances when they lend their voices to cartoons. In cases like these, animators have the additional burden of bringing aspects of an actor's persona to the way that a character behaves. Animation is sometimes reduced to being make-up or a costume applied to a live performance.
In live action, dubbing is inherently false. We're not getting the actor's voice, we're getting an approximation. Because dubbing comes after photography, the visuals drive the soundtrack. In animation, the soundtrack comes first so it drives the visuals. The animator is forced to be the servant of the voice actor, embellishing an existing performance.
Actors interpret scripts. Animators interpret actors.