My understanding is that a witness is supposed to prepare for a deposition. My first concern is how well prepared or knowledgeable for a deposition.

Suppose a material witness made a point of studying his adversary's liabilities thoroughly, and his own side's superficially. The witness then recites chapter and verse about his adversary's liability, and honestly says, "I don't know" about his side's. Can the opposing lawyer ask him, with effect, "why do you know so much about some topics, and so little about others?" Will the witness have fallen short in his duty? Can s/he be forced to learn more to be more responsive to the opposing lawyer's questions?

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    I think you should clarify what you mean by a witness. I am assuming you mean an expert witness, as this seems to make you question make sense, but it should be explicit if this is what you mean.
    – Dave
    Nov 12, 2019 at 13:18
  • @Dave: This was a material witness, not an expert, not a principal, but a friend of one of the principals, and presumably biased. See also my comment under Dale's answer.
    – Libra
    Nov 12, 2019 at 16:14

2 Answers 2


Generally speaking, a witness has no duty to "learn" anything in anticipation of a deposition. Witnesses are required to testify from their personal knowledge, which may be expansive or limited, given her intellectual capacity and preparation.

If the deposition is about a car crash, for instance, there is nothing requiring the witness to learn the layout of the intersection or the timing of the traffic lights, but nor is there any legal reason a witness could not watch 500 hours of video footage of the intersection and then testify to those things. The weight assigned to that testimony is a question for the factfinder, but legally, there is nothing improper about it.

While a witness is not required to learn information in advance of a deposition, a witness may be required to assemble information for the deposition. Under Rule 30(b)(2) of the Federal Rules of Civil Procedure, for instance, a party calling a deposition may generally require the deponent to arrive for the deposition with records or other evidence in hand.

A notable exception to the general rule is Rule 30(b)(6), which allows a party to depose an organization, such as a corporation or governmental entity. Under that rule, one party notifies the organization that it wishes to ask questions on specified subjects. The organization is then required to designate a person to testify on those subjects, and that person is required to learn all the facts "known or reasonably available to the organization" regarding those subjects.


A witness can only testify to what they witnessed

That means their testimony can only be what they personally saw or heard (or, more rarely, touched, smelled and tasted). They cannot testify to the truth of what someone else told them (although they can testify that they were told it) - that’s called hearsay and it’s inadmissible.

A witness cannot learn things after the event and testify to them.

  • I'll give you an example: In his initial statement to the authorities, a witness gave the scene of an accident as "the street that you hit going down I Street just before you reach the public library." He researches further,"learns," and at deposition, says that the "cross" street is 15th street, a fact that favors his side. He declines to do research to make similar clarifications for the other side. So he addresses one side with a certain clarity, and insultingly refers to the other side's issues as "the what you ma call its."
    – Libra
    Nov 12, 2019 at 16:19
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    The other side's lawyer can ask "Did you know at the time that it was 15th Street?" and possibly make use of the answer. A material witness is not supposed to do research about his evidence, because as Dale says it is specifically what he saw and heard, and his opinions are only relevant if they indicate bias. Nov 12, 2019 at 17:44
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    These answers are preposterous. Witnesses are perfectly free to gather additional information and testify to the facts that they learn, and their opinions can be relevant for all sorts of purposes beyond bias.
    – bdb484
    Nov 12, 2019 at 20:53
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    @bdb484: There seems to be a basic misunderstanding somewhere. Witnesses of fact testify to the facts they witnessed, only. Expert witnesses testify to their (qualified and expert) opinions. Lawyers put arguments as to the law, and the judge adjudicates on them. The jury decides who they believe on questions of fact. Going outside your area is not allowed, no matter how much you have studied. Nov 12, 2019 at 21:52
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    @TimLymingtonsupportsMonica I'm not sure what jurisdiction's rules you're working off of, but in the United States, where the question is tagged, FRE Rule 701 explicitly permits lay opinions.
    – bdb484
    Nov 12, 2019 at 23:01

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