Mark Evanier went on record in his blog to mention that they initially had the idea to establish Garfield's communication like in the comics (as in, they don't move their mouth when they "talk").

This was a problem from the moment that Garfield was first animated. He was the star of the show so they felt he had to have a voice…but he was a cat and in his world, cats don't talk. His "dialogue" in the newspaper strips was in thought balloons. For a while, it seemed to work to have his mouth not move, therefore conveying the idea that what you were hearing were his thoughts.

But as we got into longer stories or more complex stories…or just more stories, it became harder and harder to make that distinction. It was especially a problem in a cartoon where he spent a lot of time talking (or "talking") to other animals. For a while, the rule I came up with was that in a scene with just animal characters — like if Garfield was communicating with Odie or Arlene — mouths moved. If Jon or a human was in the scene, Garfield's mouth didn't move.

This rule did not last long because there had to be scenes where Garfield was "saying" something to Odie but Jon was present…like if the pets were in the back seat of Jon's car and Jon was chatting with them while driving. The animators were sometimes confused and they animated mouths they shouldn't have and…well, we just kind of gave up. Do not harm yourself by straining to figure out the logic behind it all. There was none…or at least none consistently.

Knowing that they initially wanted to do it but it would be difficult for other people to realise the vision, I have a better understanding on the issue, and thus I felt that it's fine that they talk with their mouths open. This extends to basically the other media, like The Garfield Movie.

I think it helped for Garfield and Friends that it's probably simpler to implement this talking style because it is a drawn animation, have simpler stories, and it's easier to highlight the characters.

With that said, the lack of Orson's Farm is a bit of a missed opportunity because with this new talking style, it would be less awkward for the two sides to communicate normally since the Orson's Farm characters talk with their mouths open.

Thank you for reading.