The Federal Rules of Civil Procedure
(a) In Open Court. At trial, the witnesses’ testimony must be taken in open court unless a federal statute, the Federal Rules of Evidence, these rules, or other rules adopted by the Supreme Court provide otherwise. For good cause in compelling circumstances and with appropriate safeguards, the court may permit testimony in open court by contemporaneous transmission from a different location.
(b) Affirmation Instead of an Oath. When these rules require an oath, a solemn affirmation suffices.
(c) Evidence on a Motion. When a motion relies on facts outside the record, the court may hear the matter on affidavits or may hear it wholly or partly on oral testimony or on depositions.
(d) Interpreter. The court may appoint an interpreter of its choosing; fix reasonable compensation to be paid from funds provided by law or by one or more parties; and tax the compensation as costs.