All of the human characters in the game are sort of exaggerated caricatures of real types of people and I thought about each character's individual body type when I was designing their attacks and how they would move. For instance, the large, almost gorilla-like Orderly isn't very quick and basically just lumbers down the halls and slams you with his oversized arms (he's also the only enemy the player can reasonably out run). Contrast this with the janitor who appears to be a lot more lean and agile and uses his broom as almost a sort of martial arts style staff weapon and is able to spin in the air. When plotting the janitor's movements, I actually held a broom myself and figured out exactly how I wanted him to move (though I wasn't actually able to pull off the midair spinning thing).
The enemy nurses, like the Orderlies, are very direct and not very graceful in their attacks. Because of the shape and size of the nurse's bodies, I figured that they wouldn't move around as much and would probably just have to use their arms in a limited way, in order to poke you with their giant syringe. I also took a lot of time figuring out the nurse's "run" animation and tried several different things with that. The Doctor's attacks are also very direct but he uses much wider sweeping motions that I felt were more inline with his tall lanky form. His attacks are designed to create a feeling of quick precision, which would seem to go well with a character in a white lab coat wielding a giant scalpel.
There were several other characters that we had originally planned to have in the game, but that we decide to scrap for various reasons. Among these was a security guard character that I decided was visually boring, and more importantly redundant, since we would already have both Orderlies and SWAT team in the game.
In addition to the human hospital staff, the asylum also has some enemy ghosts floating around certain sections of it. The original reason that I wanted to include ghost characters in the game is that I wanted something else strange in there other than Twisty himself. I wanted something to signify that to the asylum was actually haunted and that there was other weirdness going on there aside from the fierce floating brain.
The ghosts are the only enemies with a ranged attack that you encounter during the majority of the game, before the SWAT team enters the building. Their supernatural energy attacks gave us the opportunity to include more visual variety in the game, and depart from the standard melee weapons that the other enemies use.
The aggressive ghosts are not only the enemy of the player, but will also attack and do battle with any of the other human enemies that they encounter. Part of this design decision was based on the simple fact that it wouldn't make sense for them to get along with the "ordinary" human hospital staff, but it also serves as a cool gameplay feature that allows the player to pit different enemies against each other for their own strategic advantage.
When damaged, the ghosts leave a green ectoplasm like substance behind, as opposed to the red blood that is left by the more corporeal enemies. When designing the characters for the game, we felt it was important to give them each their own distinctive "personality". Not only does this include their individual appearances, but also the way that they move. If you look closely, even their individual movement patterns are unique and certain characters have a tendency to move forwards and backwards, strafe from side to side, or use a serpentine pattern during combat. They also have different patterns and intervals for their separate attacks during combat and leave different openings, providing "weaknesses" for the player to exploit.
All of these various factors and details add up to more interesting characters and richer gameplay.