The actors inside famous costumes: Chewbacca, Big Bird, Godzilla, and more

Big Bird/Caroll Spinney

Chewbacca/Peter Mayhew

ALF/Michu Meszaros

Barney/David Joyner

Godzilla/Haruo Nakajima

Alien/Bolaji Badejo

(via Flavorwire, where you can see a few more)