I laughed when I first heard about it. I wasn't laughing so much when I tried to act with a real guinea pig - which was what they originally wanted to try to do - you just can't get a real guinea pig to do things you want it to do.
I got pretty good at acting to a box - basically they used a box on a stick for me to look at. Then the 1st AD would read my father's lines. It was bizarre but I got used to it. Was pretty impressed by the final result of my animated father.