Each has varying speed, health, weapons skill and dark magic powers. Caleb is perhaps the most well rounded, he's the default choice. Controlling the action is pretty standard for a first person shooter. You can use the mouse to look, fire and move ahead, and the keyboard for changing your active weapon, jumping, crouching, and so on. If you don't like the default key layout, it is completely customizable and joysticks are also supported for those that prefer using them to the mouse.
Monsters are a mix of human enemies and creepy alien-monster hybrids.
The humans are generally easy to kill, they fall in a few shots. Not so all of the other monsters. The most interesting of the monsters are priests which phase in and out of reality, making timing your shots important, and a semi-invisible dude of the same type. There are also annoying Thieves, little creatures that jump on your head and suck the life out of you. Once they are on you, you can't get them off until they are done feeding.