Mirar con mouse si estas en editor o stand alone

MiraMouse.cs

using UnityEngine;
using System.Collections;

public class MiraMouse : MonoBehaviour {
    public float factorRotacion = 1;
#if UNITY_EDITOR || UNITY_STANDALONE
    Vector2 prevPos;

    void Awake()
    {
        prevPos = Input.mousePosition;
    }

    void Update()
    {
        if (Input.GetMouseButtonDown(0))
        {
            prevPos = Input.mousePosition;
        }
        else if(Input.GetMouseButton(0)) { 
            float dx = Input.mousePosition.x - prevPos.x;
            float dy = Input.mousePosition.y - prevPos.y;
            transform.Rotate(0, dx * factorRotacion, 0, Space.World);
            transform.Rotate( -dy * factorRotacion, 0, 0);
            prevPos = Input.mousePosition;
        }
    }
#endif
}