Here is some interesting info about the voice cast of American Dad! and how their lines are recorded:

The voice actors (main and presumably guest) are not assembled as a group when performing the lines of the characters; rather, each of the voice actors perform/record their lines privately. The voice actors have stated that because of their personalities and their tendency to goof off when together as a group, they would never get anything completed if they were to perform their lines collectively.

(From the Wikipedia article for "American Dad!".)