Nina Conti's Big Mouth

I've seen other ventriloquists using this device, but none were as good as Nina Conti, who completely makes you forget that she's doing all the work -- even though you can see her operating the mechanism! One of the keys to her success is that she laughs as if someone else were speaking the lines she's no doubt voiced many times before.

This should start about ten minutes in, so you see the last third of the act she performed on Russell Howard's TV show in the UK...