カメラの向きに合わせてGameObjectを操作します。 TransformDirectionを使えばGameObjectの方向ベクトルが取得できます。

using UnityEngine;
using System.Collections;
 
public class PlayerScript : MonoBehaviour
{
	CharacterController controller;
	Vector3 moveDirection;
 
	float fSpeed = 3.0f;
 
	void Start ()
	{
		controller = GetComponent("CharacterController") as CharacterController;
	}
 
	void Update ()
	{
		Vector3 forward = Camera.mainCamera.transform.TransformDirection( Vector3.forward );
		Vector3 right = Camera.mainCamera.transform.TransformDirection( Vector3.right );
		moveDirection = Input.GetAxis("Horizontal") * right + Input.GetAxis("Vertical") * forward;
		moveDirection *= fSpeed;
 
		// 移動
		controller.Move( moveDirection * Time.deltaTime );
	}
}