void LookAtPoint(int tid, fixed x, fixed y, fixed z)

Rotates the actor so that it's looking at the point. Takes the actor's ViewHeight into account.

void LookAt(int tid, int target_tid)

Rotates the actor so that it's looking at another actor. Takes into account both actors' ViewHeights to make the actor look at the other actor's head, if the target actor has a CameraHeight, or center, if it doesn't.