How to activate / deactivate an enemy on player proximity

you can use a trigger around your enemy. attach a trigger (a big one) to a gameObject you make it the children of your enemy. then attach a code like this to it.

 function OnTriggerEnter (other : GameObject)
 {
 if(other.tag == "Player") transform.parent.enabled = true;
 }
 function onTriggerExit(other : GameObject)
 {
 if(other.tag == "Player") transform.parent.enabled = false;
 ]

the code assumes that the player has a player tag assigned to it but you can identify the player in any other way. the transform.parent.enabled also enables/disables the parent of the object but you can declare a public variable and attach the enemy to it in the editor if you want. also you might want to enable/disable a script of the enemy so you can declare the variable of type "thatScript". where thatScript is the name of your script.

 var enemy : EnemyAI;

and then enable disable the enemyAI script of the object attached to this variable in editor or using GameObject.Find and GetComponent.

in this way using triggers you will not call the code in update and they are really cheap even on ios triggers don't take much. also you can make big triggers for each reagon to do enable/disable on a per reagon basis.

You wouldn't have to do it per update; you could use coroutines or InvokeRepeating to do it 10 times per second or something. It's probably better if you use large sphere triggers for the enemies, though. Then you can do

 function OnTriggerEnter () {
     enabled = true;
 }
 
 function OnTriggerExit () {
     enabled = false;
 }

Thanks, I've taken the trigger aproach, I've created a script which I attached directly to the enemy. My final (and working) code:

 private var AIScript:EnemyAI;
 
 function Awake() {
     var sc : SphereCollider;
     sc = this.gameObject.AddComponent ("SphereCollider");
     sc.radius = 50;
     sc.isTrigger = true;
     
     AIScript = GetComponent(EnemyAI);
     AIScript.enabled = false;
 }
 
 function OnTriggerEnter(other:Collider){
     if(other.tag == "Player") AIScript.enabled = true;
 }
 
 function OnTriggerExit(other:Collider){
     if(other.tag == "Player") AIScript.enabled = false;
 }

You could use a script like this:

 float activationDistance = 5f;
 GameObject enemy;
 GameObject player;
 void Update(){
 if ((enemy.transform.position - player.transform.position).magnitude < activationDistance)
 {
     enemy.enabled = true;
 }
 else
 {
     enemy.enabled = false;
 }
 }

Put this script on a parent object with the enemy as the child object because a disabled gameObject can't re-enable itself. Sorry for the bad code formatting, but it should work. Pretty reliable and should work even if the enemy travels too fast to cause problems with colliders and things, since it's based on just the enemy's distance from the player. In my opinion, this is a more reliable system and a bit simpler too.