So, how do you do that? Find a talking dog? Train your dog to talk? Better yet, train your dog to move it's mouth on cue? The technique is a lot simpler than these suggestions. First begin by noticing an animal that moves it's lips or body in such as way that it appears to be engaged with the camera, yourself, or someone/thing you are filming. I use my goldfish since his mouth moves, and "he" looks toward me when I move close to his tank.
Record a short scene. It is not usually necessary to work hard on the action or direct the animal. From that short movie, imagine what the animal could say and script it according to the lip movements. Say the words into the mirror and observe your own lip movements, if that helps. Then fill in the response dialogue that could be made from yourself. Look for a good story ending that involves the animal reacting to the story or situation. Finally, record your human script and sync it to the actions of the animal. Voila! Talking animal. See more examples on YouTube.http://www.youtube.com/watch?v=nGeKSiCQkPw
PS- The dog in this movie does not really talk. ;-)
Some examples from BBC are available here. (Warning: Some content may be questionable for certain student school-based audiences. Always preview before showing.)