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.