ArgumentException: You are not allowed to call GetComponent when declaring a variable.
Move it to the line after without a variable declaration.
Don't use this function in the constructor or field initializers, instead move initialization code to the Awake or Start function.
steps..ctor () (at Assets/steps.js:8)

var controller : CharacterController = GetComponent(CharacterController);
var FootSteps : AudioClip [];
private var StepTime : float = 0;
var audioSource : AudioSource;
private var stepping : boolean = false;

function Update () {
if(controller.isGrounded){
if(!stepping){

if (Input.GetButton("Vertical") || Input.GetButton("Horizontal")){
StepTime = 0.5;

}

}
}
}

function footfall(){
stepping = true;
audioSource.PlayOneShot (FootSteps[Random.Range(0,FootSteps.length)]);
yield WaitForSeconds (StepTime);
stepping = false;
}

var controller : CharacterController;
            
          controller = gameObject.GetComponent(CharacterController);
var FootSteps : AudioClip [];    // Таблица звуков.
private var StepTime : float = 0;        // Время шага.
var audioSource : AudioSource;
private var stepping : boolean = false; // Шагаем? По умолчанию нет.

function Update () {
if(controller.isGrounded){   // если персонаж на земле тогда...
if(!stepping){               // если персонаж не шагает тогда...

if (Input.GetButton("Vertical") || Input.GetButton("Horizontal")){   // нажата ли клавиши ходьбы.
StepTime = 0.5;              // назначаем время одного шага.
footfall();

}   

}
}
}

function footfall(){
   stepping = true;
   audioSource.PlayOneShot (FootSteps[Random.Range(0,FootSteps.length)]);
   yield WaitForSeconds (StepTime);   
   stepping = false;
}

NullReferenceException: Object reference not set to an instance of an object
steps.Update () (at Assets/steps.js:17)