Non-Testifying Expert Discovery: Rule 26, Identity Disclosure, and Exceptions

A non-testifying expert, often called a consulting expert, may help counsel assess technical issues, test case theories, review another expert's work, or identify evidentiary weaknesses without being expected to testify at trial. That role can offer important strategic value, but it also creates distinct discovery questions.

Under the federal rules, discovery of facts known or opinions held by a retained consulting expert is limited in specified circumstances. The limitation is not absolute, and the treatment of an expert's identity may vary by jurisdiction. Attorneys should evaluate the governing federal, state, and local rules before assuming a consulting engagement will remain confidential.

This article addresses federal civil discovery principles, particularly Federal Rule of Civil Procedure 26(b)(4)(D). State rules and local practice may differ. For a broader discussion of role selection, see consulting vs. testifying experts.

How Discovery Rules Apply to Non-Testifying Experts

Rule 26 distinguishes between experts expected to present opinions at trial and experts retained or specially employed in anticipation of litigation or trial preparation who are not expected to testify. A consulting expert may be engaged to evaluate records, inspect evidence, assist in selecting a testifying expert, or pressure-test a proposed opinion. The expert's work may guide litigation strategy even if it is never introduced in court.

The distinction matters because a testifying expert generally triggers broader disclosure obligations. A consulting expert's facts and opinions are subject to the more limited framework in Rule 26(b)(4)(D), provided the expert fits within the rule and is not expected to be called as a witness.

The designation is not merely a label. Discovery treatment can turn on the expert's actual function, the materials shared, later disclosures, claims or defenses asserted in the case, and the governing jurisdiction's authority. If a consulting expert later becomes a testifying witness, the applicable disclosure analysis can change substantially.

• Expert role: Key Question: Is the expert expected to testify?; Why It Matters: Testifying experts generally face broader disclosure requirements.
• Facts and opinions: Key Question: Was the expert retained for litigation or trial preparation?; Why It Matters: Rule 26(b)(4)(D) may limit discovery.
• Equivalent information: Key Question: Can the requesting party obtain it by other means?; Why It Matters: Practical availability is central to exceptional-circumstances arguments.
• Identity: Key Question: Does controlling authority protect the consultant's identity?; Why It Matters: Courts have taken differing approaches.

What Discovery Is Limited Under Rule 26(b)(4)(D)

Federal Rule 26(b)(4)(D) provides that, ordinarily, a party may not discover facts known or opinions held by an expert who has been retained or specially employed by another party in anticipation of litigation or trial preparation and is not expected to testify. The rule identifies two routes for discovery: as provided in Rule 35(b), concerning certain physical and mental examinations, or upon a showing of exceptional circumstances under which it is impracticable to obtain facts or opinions on the same subject by other means.

This is a discovery limitation, not a blanket privilege. It should not be conflated with attorney-client privilege or work-product protection. Those doctrines have different elements, purposes, and waiver analyses. Depending on the facts and jurisdiction, communications involving counsel and a consulting expert may implicate those doctrines, but Rule 26(b)(4)(D) itself does not convert every consulting-expert communication or document into privileged material.

The practical question is often narrower than whether the consultant's entire file is protected. Counsel may need to assess the expert's role, the purpose of the engagement, the specific facts or opinions sought, whether the information is available elsewhere, and whether any conduct has placed the consultant's work at issue. For a separate discussion of these confidentiality doctrines, see consulting expert privilege and confidentiality.

Exceptional Circumstances: The Federal Standard

Exceptional circumstances under Rule 26(b)(4)(D) are fact-dependent. The rule focuses on whether it would be impracticable for the requesting party to obtain facts or opinions on the same subject by other means. The availability of equivalent information, the condition of relevant evidence, timing, cost, access, and the practical ability to conduct an independent evaluation may all matter depending on the dispute.

The standard does not create a categorical result whenever a consulting expert inspected evidence or formed an unfavorable opinion. Nor does the fact that an expert's work would be useful establish exceptional circumstances. Courts examine the particular record, including whether a party has meaningful access to comparable facts or opinions without invading the opposing party's consulting relationship.

Attorneys should therefore avoid treating a consulting designation as an automatic shield. At the outset of an engagement, it is useful to consider what evidence the consultant will examine, whether the evidence may later become unavailable or altered, and what a court could regard as an alternative source of substantially equivalent information. That analysis should be coordinated with case-specific discovery strategy.

Is a Consulting Expert's Identity Discoverable?

Rule 26(b)(4)(D) expressly addresses discovery of facts known and opinions held by qualifying non-testifying experts. It does not expressly resolve whether the identity of every consulting expert is itself protected. Courts have reached different conclusions, so identity disclosure requires jurisdiction-specific research.

Authority Supporting Protection of Identity

Some decisions have read the Rule 26 limitation to extend to the identity of a non-testifying consulting expert. In *Liverperson, Inc. v. 24/7 Customer, Inc.*, No. 14 Civ. 1559 (S.D.N.Y. July 29, 2015), the court held that a non-testifying expert's identity was protected from disclosure absent exceptional circumstances. The decision cited authority taking a similar approach.

In Williams v. Bridgeport Music, Inc., 300 F.R.D. 120, 122 (S.D.N.Y. 2014), the court precluded discovery of the identity of an informal consulting expert. In Dover v. British Airways, PLC (UK), 2014 WL 4065084, at *1 (E.D.N.Y. Aug. 15, 2014), aff'd, 2014 WL 5090021 (E.D.N.Y. Oct. 9, 2014), the court likewise addressed protection for a consulting expert, while noting that a party may create disclosure issues by putting the consultant's work at issue.

The Tenth Circuit stated in *Ager v. Jane C. Stormont Hospital and Training School for Nurses*, 622 F.2d 496, 500-01 (10th Cir. 1980), that the rule's preclusion of discovery against non-testifying experts “not only encompasses information and opinions developed in anticipation of litigation, but also insulates discovery of the identity and other collateral information concerning experts consulted informally.”

Authority Permitting Identity Disclosure

Other courts have allowed discovery of a consulting expert's identity without requiring the same showing necessary to obtain the expert's facts or opinions. In *Baki v. B.F. Diamond Construction*, 71 F.R.D. 179 (D. Md. 1976), the court concluded that identifying information could be obtained through properly framed interrogatories. The court stated:

[T]he names and addresses, and other identifying information, of experts, who have been retained or specially employed in anticipation of litigation or preparation for trial and who are not expected to be called as witnesses at trial, may be obtained through properly framed interrogatories without any special showing of exceptional circumstances in the absence of some indication that such information by reason of facts peculiar to the case at issue, is irrelevant, privileged, or for some other reason should not be disclosed.

Other decisions have followed or found persuasive the Baki analysis. See *Arco Pipeline Co. v. S/S Trade Star*, 81 F.R.D. 416, 417 (E.D. Pa. 1978); Threlkeld v. Haskins Law Firm, 1990 WL 124876, at 1 (E.D. La. Aug. 21, 1990); and Lesclard v. Babcock & Wilcox, 1990 WL 124297, at 1 (E.D. La. Aug. 9, 1990).

These authorities demonstrate why a party should not assume that identity disclosure is governed by a single national rule. Before responding to an interrogatory, serving one, or structuring an engagement around anticipated confidentiality, counsel should review controlling circuit and district authority, local rules, and the procedural posture of the case.

Practice Considerations Before Retention

The discovery analysis should begin before a consultant is engaged. Counsel can reduce avoidable uncertainty by identifying the expert's anticipated role and documenting the engagement consistently with the litigation strategy.

Useful considerations include:

• Determine whether the expert is being retained solely for consulting work or may later be designated to testify.
• Clarify the scope of the engagement, including whether the expert will inspect evidence that may be difficult for an opposing party to examine independently.
• Consider potential discovery requests involving the expert's identity, work, communications, and materials reviewed.
• Preserve records and communications in a manner consistent with counsel's legal obligations and the governing discovery rules.
• Reassess the expert's role if the litigation strategy changes or if the expert's work may be affirmatively relied upon.

A consulting expert can provide valuable technical judgment, including assistance vetting a testifying expert or reviewing opinions that may face a Daubert challenge. But the value of that work depends in part on understanding the discovery consequences of the engagement from the start.

For retention, communication, and project-management considerations beyond the discovery analysis, see practical steps for working with a non-testifying expert.