N.C. Gen. Stat. § 84-11
Every attorney who claims to enter an appearance for any person shall, upon being required so to do, produce and file in the clerk's office of the court in which he claims to enter an appearance, a power or authority to that effect signed by the persons or some one of them for whom he is about to enter an appearance, or by some person duly authorized in that behalf, otherwise he shall not be allowed so to do: Provided, that when any attorney claims to enter an appearance by virtue of a letter to him directed (whether such letter purport a general or particular employment), and it is necessary for him to retain the letter in his own possession, he shall, on the production of said letter setting forth such employment, be allowed to enter his appearance, and the clerk shall make a note to that effect upon the docket.
N.C. Gen. Stat. § 84-11