In the demo, Fred was equipped with his default jar head, as well as a hulking stone idol head (which changed his physique into that of a lumbering gorilla), and a rotting "corpse head" that turned him into a zombie. The stone head was good for pushing heavy objects, smashing through walls and other feats of strength, but the corpse head offered more elegant solutions. It can be filled with just about any liquid (which makes it all big and gross and wobbly), which enables Fred to do things like suck water out of fishtanks or broken pipes to put out fires, or fill it full of gasoline, which turns him into a flamethrower if he spits it out near an open flame. Stand in an updraft, and it'll inflate with air, launching you straight up and enabling you to float to a far-off ledge.
Interestingly, though, it's the plain old default jar head that turned out to be the most effective in clearing out the Bone Thugs that stood between us and all the puzzles. That's partly because the game's brawler-style combat follows a rock-paper-scissors dynamic, where one head type is more powerful against certain types of foes. Wearing the right head will also enable you to counter your enemies' unblockable super moves, resulting in a wickedly brutal fatality - which is a hell of a lot more entertaining to perform than the standard, yanking decapitations Fred executes when he's beaten up a monster enough.
As an interesting aside, Dead Head Fred has also found a way to be a deep brawler without compromising all the things you can do in the rest of the game. There's only one attack button, but landing a successful hit turns all the other buttons into attack buttons, too, opening up possibilities for all sorts of interesting combos. When you consider that switching to different heads opens up even more moves and fighting styles, and that each of those heads can be upgraded, there should be enough here to keep jaded fans of punching and kicking happy.