How do I move a CharacterController type in the direction it's facing?

I should mention this is an NPC and needs to operate without input from the player. I'd like to drop this NPC somewhere in the game and then use the Editor's rotate tool to point him in the direction I want him to walk forward.

My Code:

 function FixedUpdate()
 {
   moveDirection = Vector3.forward;
   moveDirection *= speed;

   // Apply gravity
   moveDirection.y -= gravity * Time.deltaTime;

   // Move the controller
   var controller : CharacterController = GetComponent(CharacterController);
   var flags = controller.Move(moveDirection * Time.deltaTime);
   grounded = (flags & CollisionFlags.CollidedBelow) != 0;
 }

When the game runs, my character moves in the Vector3.forward direction, however if I rotate him to point in a different direction (lets say 45 degrees) and start the game, then he's facing 45 degrees but is still moving in Vector.forward. I'm guessing the rotation isn't changing his Z-axis(?)...

The only thing I could think of is to take the rotation angle he's facing and figure out a way to calculate how much Z and how much X to put in his "moveDirection." For example if his rotation is 45 degrees then he's in the middle between the Z and X axis; so that's Vector3(0.5,0,0.5) 50% X-axis and 50% Z-axis. I'm guessing there's an easier way then trying to compute it manually...

I tried another solution on Unity Answers that suggested to use "transform.LookAt" but his solution didn't work for me: http://answers.unity3d.com/questions/5364/how-can-i-get-my-model-to-face-the-direction-it-is-moving

Thank you in advance!