The voice actors were so good the show was nearly impossible to animate

One of the things I've noticed watching the show again is that Billy West, Fred Newman, Connie Schulman, etc. often put so much humanity into their characters that the audio track didn't sound like what you'd expect to hear for a cartoon audio track. If I heard the audio and wasn't looking at the screen and didn't know what it was, I'd assume it had to be some kind of live action show.

Most cartoons, especially these days, are tongue-in-cheek in delivery throughout. Da-da da-da da-da da-da, dadadadada. Set up lines, then the punchlines. Playful interaction with little-to-no recognizable humanity. This makes it really easy for animators, since the characters don't interact or emote the way real people do. They can convey it as abstractly as they want.

The readings for this show were done in such a way that you'd really only convey that kind of emotion through live actors; there were too many nuances/subtleties in the voice acting to match with drawing. When Billy West does a Doug reading, there are often a million different things going on at once.

It's something I have to get used to with time whenever I watch the series: Hearing these incredibly real people but seeing that it's coming from cartoon characters.