In the U.S., a party may hire experts to help them evaluate a given case. For example, a car maker may hire an experienced mechanic to decide if its cars were built to specification. This kind of expert opinion will be protected from discovery by the opposing party. In other words, if the expert finds evidence against their client, the opposite party will not automatically gain access to it. This privilege is similar to the work-product doctrine (not to be confused with attorney–client privilege).
The non-testifying expert can be present at the trial or hearing to aid the attorney in asking questions of other expert witnesses. Unlike a testifying expert, a non-testifying expert can be easily withdrawn from a case. It is also possible to change a non-testifying expert to a testifying expert before the expert disclosure date.