Enabled Behaviours are Updated, disabled Behaviours are not.
This is shown as the small checkbox in the inspector of the behaviour.
#pragma strict
// Required when Using UI elements.
public class Example {
	public var pauseMenu;
	public function Start() {
		//Enables the pause menu UI.
		pauseMenu.enabled = true;
	}
}
using UnityEngine; using System.Collections; using UnityEngine.UI; // Required when Using UI elements.
public class Example : MonoBehaviour { public Image pauseMenu; public void Start() { //Enables the pause menu UI. pauseMenu.enabled = true; } }