From LegalLanding

A deposition, allowed by FRCP Rule 30, is the most expensive form of discovery in terms of overall litigation expenditure. The questioner can compel witnesses to answer questions fairly spontaneously, and then can follow up on the answers with further questions. Anything transcribed by the reporter is available for use in the case as though it occurred in open court (to the extent that it is admissible).

Any party can schedule a deposition on “reasonable notice” in writing. If the witness is a party, then only notice is necessary; for nonparties, a subpoena must be served. There is a numerical limit of ten depositions per side, and each is limited to “one day of seven hours,” although the seven hours can be split up over multiple days with the other party’s consent.

The witness’s attorney can object when questions are improper, but the examination will proceed; the testimony is taken subject to any objections. In the past, objections were normally used as part of the “woodshedding” process of the witness, to direct him to answer (or not) in a given manner. However, no off-the-record consultation between lawyer and witness is allowed during the deposition, including recesses, except to determined if a privilege applies (see Hall v. Clifton Precision (1993)).

Any deposition can be used for impeachment at trial.

Depositions by Written Questions are also allowed by FRCP Rule 31. The chief benefit of a deposition is the ability to ask open-ended questions to further discovery; that benefit is gone with this tool. It might use when there is a long distance between the questioner and the witness (although now video-conferencing is allowed by the rules). When this is necessary, the only benefit of the written deposition is that the witness cannot have his lawyer answer the questions.