The Alien is leashed to the player at all times. On harder difficulties the leash becomes tighter and the Alien is more responsive. Personally, I don't like the AI because it makes gameplay become too routine and it subsequently kills any proper tension. It can sometimes even get to the point where it's just annoying. However, in my experience the Alien is less predictable on the easier difficulties because it's a bit more random in how he responds, which can make gameplay more fun.