Peter Tork Mouthing Everyone Else's Lines

When a ventriloquist provides a voice for his dummy, the illusion works because you are focused on the dummy delivering the lines and you're not looking at the ventriloquist. In the age of video, you can go back and focus on the ventriloquist and see his/her lips moving. That's the way you should watch this compilation- focus on Peter Tork's lips, not who is actually speaking. 

Tork had a habit of mouthing lines that weren't his while shooting the sitcom The Monkees. According to comments at Metafilter, this is very common among beginning actors, because it helps with timing and catching your cue. The real trick is to learn how to keep up with memorized dialogue without moving your lips. As it is, we rarely see it because we focus on the actual speaker. 

OR... was Tork actually delivering all the lines in the show, while everyone else just moved their lips? We may never know.