40 C.F.R. § 27.33 – Witnesses.

(a) Except as provided in paragraph (b) of this section, testimony at the hearing shall be given orally by witnesses under oath or affirmation.
(b) At the discretion of the presiding officer, testimony may be admitted in the form of a written statement or deposition. Any such written statement must be provided to all other parties along with the last known address of such witness, in a manner which allows sufficient time for other parties to subpoena such witness for cross-examination at the hearing. Prior written statements of witnesses proposed to testify at the hearing and deposition transcripts shall be exchanged as provided in § 27.22(a).
(c) The presiding officer shall exercise reasonable control over the mode and order of interrogating witnesses and presenting evidence so as to:
1. (1) Make the interrogation and presentation effective for the ascertainment of the truth,
2. (2) Avoid needless consumption of time, and
3. (3) Protect witnesses from harassment or undue embarrassment.
(d) The presiding officer shall permit the parties to conduct such cross-examination as may be required for a full and true disclosure of the facts.
(e) At the discretion of the presiding officer, a witness may be cross-examined on matters relevant to the proceeding without regard to the scope of his or her direct examination. To the extent permitted by the presiding officer, cross-examination on matters outside the scope of direct examination shall be conducted in the manner of direct examination and may proceed by leading questions only if the witness is a hostile witness, an adverse party, or a witness identified with an adverse party.
(f) Upon motion of any party, the presiding officer shall order witnesses excluded so that they cannot hear the testimony of other witnesses. This rule does not authorize exclusion of—
1. (1) A party who is an individual;
2. (2) In the case of a party that is not an individual, an officer or empoyee of the party appearing for the entity pro se or designated by the party's representative; or
3. (3) an individual whose presence is shown by a party to be essential to the presentation of its case, including an individual employed by the Government engaged in assisting the representative for the Government.