Assignment working in Update() but not in Start()?

I'm trying to access my Player object's Rigidbody2D so I can set its velocity based on controller input. To do this, I was assigning the Rigidbody2D to a variable "playerRB", using the following line of code:

 Rigidbody2D playerRB = this.gameObject.GetComponent<Rigidbody2D> ();

After some trial and error, I found that my approach works just fine but only if I place that assignment statement in the Update() function. I'd rather place it in the Start() function, because wouldn't that be more efficient? As I understand it, it shouldn't need to reassign that value every frame, and doing so is just wasting CPU time. Maybe I'm wrong, though, and that assignment does need to happen every frame.

Is it not working in Start() because the object isn't yet initialized so trying to access the GameObject at that point doesn't work? If so, can I create a separate function to handle the assignment of playerRB, and set that function to run only once immediately after the object is initialized?

Full code below. Note that the code pasted below is working as intended, I just want to know if it's possible to move the assignment of playerRB from Update() to Start() to be more efficient. I'm new to this, so any other comments, criticisms, suggestions or improvements are also welcome.

 using UnityEngine;
 using System.Collections;
 
 public class PlayerController : MonoBehaviour {
 
     public float Speed = 1.0f;
     float velx = 0.0f;
     float vely = 0.0f;
     public PlayerProjectile weapon;
     Rigidbody2D playerRB;
 
     // Use this for initialization
     void Start () 
     {
         //The line below doesn't work when placed here in Start(), but it works when it's placed in Update ().
         //Rigidbody2D playerRB = this.gameObject.GetComponent<Rigidbody2D> ();
     
     }
     
     // Update is called once per frame
     void Update () 
     {    
         //The line below works when placed in Update(), but I'd prefer not to call it every frame.
         Rigidbody2D playerRB = this.gameObject.GetComponent<Rigidbody2D> ();
         ControllerInput ();
         Vector2 movement = new Vector2 (velx, vely);
         if (playerRB == null) 
         {
             Debug.Log ("playerRB is null!");
         }
 
         playerRB.velocity = (movement * Time.deltaTime * Speed);
     }
     void ControllerInput()
     {
         velx = Input.GetAxis ("LeftStick_X");
         vely = Input.GetAxis ("LeftStick_Y");
         if (Input.GetButtonDown ("Y"))
             {
             weapon = Resources.Load<PlayerProjectile>("Prefabs/Weapons/FastZombieCannon");
             }
         if (Input.GetAxis ("RightTrigger") >= 0.5f) 
         {
             Fire();
         }
     }
 
     void Fire()
     {    
         float playerDirection = Mathf.Atan2 (Input.GetAxis ("RightStick_Y"), Input.GetAxis ("RightStick_X"));
         PlayerProjectile playerProj = Instantiate (weapon, new Vector3 (this.gameObject.transform.position.x + (   Mathf.Cos (playerDirection)  * 1.0f     ), this.gameObject.transform.position.y + (     Mathf.Sin (playerDirection)  * 1.0f     ), 0), Quaternion.identity) as PlayerProjectile;
         //playerProj.transform.rotation = Quaternion.Euler (playerDirection, 0, 0);
         Rigidbody2D projRB = playerProj.gameObject.GetComponent<Rigidbody2D> ();
         if (projRB == null) 
         {
             Debug.Log ("playerProj Rigidbody2D not found!");
         }
         
         projRB.velocity = new Vector2 (  (   Mathf.Cos (playerDirection)  * playerProj.ProjectileSpeed   ) , (     Mathf.Sin (playerDirection)  * playerProj.ProjectileSpeed     ));
     } 
 
 }