During questioning, it is not inappropriate for the expert to refer to his report or notes if necessary to accurately relate data or detail. To do so, permission must be sought from the judge. The expert may simply state, "I need to refer to my report for that detail; may I do so?"
FRE 612 permits such a procedure and allows witnesses to "refresh their recollection" when they cannot fully recall a fact or an event. Any document, or other item used to refresh recollection or testify from, must be disclosed to the opposing attorney.
If the expert simply can't remember the fact or opinion, even after reviewing the relevant report(s), there may still be a way to present the information to the jury.
The expert's report may constitute a record of a regularly conducted activity (commonly known as a (business record) or, if it was an accurate memorandum prepared or adopted when the facts were fresh in the expert's mind, the report may be admitted as a hearsay exception called "past recollection recorded." The admissibility argument will come from the attorney. The important point is for the expert to be clear about when there is a memory lapse or gap.
Regardless of any memory lapse, the expert's curriculum vitae and report will often be marked and accepted as trial exhibits.
However, the contents of the report will not necessarily be reviewable by the jury during deliberations, again making necessary a clear verbal articulation of the expert's findings and the facts that support them.