Adding gravity to NPC, such as an enemy or animal?

When I place my enemy into the scene and play the enemy follows be by floating around and doesn't stick to the ground. Here is my code:

 var Distance;
 var Target : Transform;
 var lookAtDistance = 25.0;
 var chaseRange = 15.0;
 var attackRange = 1.5;
 var moveSpeed = 5.0;
 var Damping = 6.0;
 var attackRepeatTime = 1;
 var TheEnemyDamage = 40;
 
 private var attackTime : float;
 
 var controller : CharacterController;
 var gravity : float = 20.0;
 private var MoveDirection : Vector3 = Vector3.zero;
 
 
 function Start()
 {
     attackTime = Time.time;
 }
 
 
 function Update ()
 {
     Distance = Vector3.Distance(Target.position, transform.position);
     
     if (Distance < lookAtDistance)
     {
         lookAt();
     }
     
     if (Distance > lookAtDistance)
     {
         renderer.material.color = Color.green;
     }
     if (Distance < attackRange)
     {
         attack();
     }
     else
     if (Distance < chaseRange)
     {
         chase();
     }
 }

That is not all of it but can someone tell me if I should include the whole script.

I also tried adding a rigid body to it but that wont allow the enemy to move at all.