When faced with an audit you may act on your own behalf at the interview or you may be represented. Tax audit representation is very important to select. For example you may want to be accompanied by a tax audit attorney, a CPA, or an EA (enrolled agent), or the person who prepared the return and signed it as the preparer. Treas. Reg. Sec 601.105(b)(1).
For the service to recognize a person as your representative, the person must display written authorization from you, the taxpayer. The authorization is generally made on Form 2848, the IRS version of a power of attorney. Or, any other proper authorization may be used. IRC Sec. 7521(c). Ordinarily, for your tax attorney or other representative to view confidential tax information belonging to you there must be an authorization signed by you that specifies what information may be viewed and acquired. Even if your representative is representing you only by letter in a letter audit there still must be a proper written authorization filed with the IRS.