At some point in the show William gets shot at, it hurts but it doesn't break the skin, so somehow the bullets have a variable speed controlled by computers.
There's also a short scene in the show where two humans are talking. One of them takes his knife out in an intimidating way and in a split second the closest host just takes the sharp part of the knife in his hand and put it in the table, so even melee weapons can't hurt real humans.
