Fact discovery and expert discovery are the two sequential phases of pretrial investigation in federal civil litigation, and they do different jobs. Fact discovery gathers the raw evidence about what happened: documents, communications, and testimony from people with firsthand knowledge. Expert discovery comes later and brings in specialists who interpret that evidence and offer opinions the jury couldn’t reach on its own. Different rules, different tools, different deadlines, and different protections apply to each.
The Federal Rules of Civil Procedure structure the two phases so that the factual record is largely built before experts weigh in on it. Understanding which tools belong to which phase, and what each side owes the other at each stage, is what keeps evidence admissible at trial.
What Fact Discovery Covers
Fact discovery is about assembling the underlying record. The federal rules allow discovery of any non-privileged matter relevant to a claim or defense, as long as the effort is proportional to the needs of the case.1Legal Information Institute. Federal Rules of Civil Procedure Rule 26 – Duty to Disclose; General Provisions Governing Discovery Before either side even sends a request, Rule 26(a)(1) requires automatic initial disclosures covering people with discoverable information, supporting documents, a damages computation, and any applicable insurance policy.
From there, four tools do most of the work.
Interrogatories
Interrogatories are written questions the opposing party must answer under oath. They’re useful for pinning down basic facts: who was involved, when events occurred, where records are kept. Rule 33 caps each side at 25 interrogatories, including subparts, unless the court or the parties agree otherwise.2Legal Information Institute. Federal Rules of Civil Procedure Rule 33 – Interrogatories to Parties That cap forces careful drafting.
Requests for Production
Rule 34 lets a party demand documents, electronically stored information, and tangible items from the other side. Emails, text messages, contracts, financial records, and digital files all fall within reach.3Legal Information Institute. Federal Rules of Civil Procedure Rule 34 – Producing Documents, Electronically Stored Information, and Tangible Things Cases with years of email archives or large databases often require specialized search tools and substantial expense.
Depositions
Depositions are live oral questioning under oath. Unlike interrogatories, they allow real-time follow-up and let attorneys observe a witness’s demeanor. Each side is limited to 10 depositions, and each one to a single day of seven hours, absent a court order or agreement.4Legal Information Institute. Federal Rules of Civil Procedure Rule 30 – Depositions by Oral Examination The noticing party chooses the recording method; any party wanting a different format must give notice and pay for it.
Requests for Admission
Requests for admission force the opposing party to admit or deny specific factual statements. If the party doesn’t respond within 30 days, the matter is deemed admitted and is conclusive for the rest of the case. It’s an efficient way to narrow the issues before trial.
Fact discovery also reaches beyond the parties. Rule 45 subpoenas can command a non-party to testify, produce documents, or permit inspection, generally within 100 miles of where the person lives or works.5Legal Information Institute. Federal Rules of Civil Procedure Rule 45 – Subpoena Before serving a non-party subpoena for documents, the requesting party must give notice to every other party in the case.
What Expert Discovery Covers
Expert discovery begins once the factual record is largely in place. It introduces witnesses who can offer opinions a lay witness couldn’t. The rules treat those witnesses differently depending on how they came into the case.
Retained Experts and Written Reports
An expert retained or specially employed to provide testimony must be identified and must submit a written report under Rule 26(a)(2)(B). The report has to contain every opinion the expert will express with the reasoning behind each, the facts and data considered, any supporting exhibits, the expert’s qualifications and publications from the previous 10 years, a list of cases in which the expert testified at trial or by deposition during the previous four years, and a statement of compensation.1Legal Information Institute. Federal Rules of Civil Procedure Rule 26 – Duty to Disclose; General Provisions Governing Discovery The expert prepares and signs the report; it is not written by counsel.
The prior-testimony list often supplies cross-examination ammunition. An expert who has testified repeatedly for one side can look less like a neutral analyst and more like a paid advocate.
Non-Retained Experts
Not every expert is a hired specialist. Treating physicians, company engineers, and other employees whose regular duties don’t involve testifying may still have relevant specialized knowledge. These non-retained experts don’t have to prepare a full written report. Instead, the party must disclose the subjects on which the witness will offer opinions and a summary of the facts and opinions to which they’ll testify. A treating physician can testify about what they observed and the treatment they provided without producing the report a retained expert must submit. The line gets blurry when that physician starts offering broader opinions on causation or long-term prognosis.
Expert Depositions
After the report is served, the opposing side gets to depose the expert. The same seven-hour default applies, though courts often allow more time for complex technical testimony.4Legal Information Institute. Federal Rules of Civil Procedure Rule 30 – Depositions by Oral Examination Deposing counsel typically probes the assumptions behind the analysis, whether the methodology is generally accepted, how the expert dealt with data that cut against their conclusions, and how their compensation compares to their independence.
The party taking the deposition must pay the expert a reasonable fee for the time spent, unless doing so would cause manifest injustice. Rates commonly run several hundred dollars per hour, and above $1,000 per hour is not unusual for physicians or highly specialized fields. Deposing several experts can add tens of thousands of dollars to the cost of a case.
How the Two Phases Differ at a Glance
Once the mechanics are on the table, the comparison sharpens on four axes.
Sequencing. Fact discovery runs first and generally closes before expert disclosures come due. The default federal rule sets expert disclosures at least 90 days before trial, with rebuttal expert disclosures due within 30 days after the other side’s expert disclosure.1Legal Information Institute. Federal Rules of Civil Procedure Rule 26 – Duty to Disclose; General Provisions Governing Discovery Court scheduling orders almost always set specific dates, and those dates often fall earlier than the default.
What each side must produce. Fact discovery works on a request-and-respond model after the initial Rule 26(a)(1) disclosures. Expert discovery requires affirmative, detailed disclosures: a full written report for retained experts, a subject-and-summary disclosure for non-retained experts.
Who the witnesses are. Fact witnesses testify about what they saw, heard, or did. Expert witnesses testify to opinions based on specialized knowledge. A treating physician can wear both hats depending on the question.
What gets admitted at trial. Factual evidence needs only to be relevant under Rule 401 and not excluded under Rule 403. Expert testimony has to clear an additional reliability gate.
Work Product Protections That Only Apply to Experts
The 2010 amendments to the federal rules added protections around expert work that don’t have a parallel in fact discovery.
All drafts of an expert’s report or disclosure are shielded as work product under Rule 26(b)(4)(B), whatever the format. Word documents, handwritten notes, and tracked-changes emails all fall within the protection, and the opposing party generally cannot obtain them.1Legal Information Institute. Federal Rules of Civil Procedure Rule 26 – Duty to Disclose; General Provisions Governing Discovery
Communications between counsel and a retained expert are also protected as work product, with three exceptions. The opposing side can still discover communications that relate to the expert’s compensation, that identify facts or data the attorney provided for the expert to consider, or that identify assumptions the attorney told the expert to rely on. Everything outside those three categories stays privileged, which lets counsel and expert have candid strategy conversations without every email surfacing as a trial exhibit.
Consulting experts hired only to help counsel understand the case, with no plan to have them testify, get the strongest protection. Their identity, opinions, and mental impressions generally cannot be discovered unless exceptional circumstances make it impossible to get the same information any other way.
Fact discovery has no equivalent shielding for early drafts of documents or for internal communications about the facts. Privilege applies where it applies by ordinary rules; there is no draft-report analog.
Deadlines and Consequences for Missing Them
The scheduling order the court issues early in the case sets separate cutoffs for fact discovery and expert discovery. Fact discovery deadlines govern interrogatories, document requests, depositions, and other factual investigation. Expert deadlines govern disclosures, reports, and expert depositions. The phases are sequential, though they can overlap depending on the schedule.
Rule 26(e) imposes a continuing duty to supplement or correct both fact and expert disclosures when a party learns that a prior response is materially incomplete or incorrect.1Legal Information Institute. Federal Rules of Civil Procedure Rule 26 – Duty to Disclose; General Provisions Governing Discovery For experts, the duty extends to both the written report and anything said during the deposition, and any corrections must be made by the time pretrial disclosures are due.
The consequences for failing to disclose or supplement can be steep. A party that fails to provide information or identify a witness as required by Rule 26(a) or (e) generally cannot use that information or witness at trial unless the failure was substantially justified or harmless.6Legal Information Institute. Federal Rules of Civil Procedure Rule 37 – Failure to Make Disclosures or to Cooperate in Discovery; Sanctions Rule 37 also authorizes courts to exclude evidence, give adverse-inference instructions, impose monetary sanctions including attorney fees, or in extreme cases enter default judgment or dismiss the claims. Missing an expert disclosure deadline can be catastrophic, because exclusion of the expert is often the direct remedy.
Admissibility After Discovery Ends
Getting information through discovery is not the same as getting it in front of the jury. The admissibility standards diverge along the same fact/expert line.
For factual evidence, the test is relevance. Under Rule 401 of the Federal Rules of Evidence, evidence is relevant if it has any tendency to make a fact in the case more or less probable.7Legal Information Institute. Federal Rules of Evidence Rule 401 – Test for Relevant Evidence Rule 402 makes relevant evidence generally admissible and irrelevant evidence inadmissible.8Legal Information Institute. Federal Rules of Evidence Rule 402 – General Admissibility of Relevant Evidence Rule 403 lets the court exclude even relevant evidence if its value is substantially outweighed by the risk of unfair prejudice, jury confusion, or waste of time.9Legal Information Institute. Federal Rules of Evidence Rule 403 – Excluding Relevant Evidence for Prejudice, Confusion, Waste of Time, or Other Reasons
Expert testimony has to clear an additional gate. The Daubert standard requires the trial judge to evaluate whether the expert’s methodology is both relevant and reliable before the testimony reaches the jury, considering factors such as whether the theory or technique has been tested, subjected to peer review, has a known error rate, and is generally accepted in the relevant scientific community. The Supreme Court’s decision in Kumho Tire extended this gatekeeping role beyond scientific experts to all expert testimony, including engineers, accountants, and other technical specialists.10Legal Information Institute. Daubert Standard
One boundary worth noting: not every state follows Daubert. California, New York, Pennsylvania, and Illinois, among others, still apply the older Frye standard, which asks only whether the methodology is generally accepted in the relevant scientific community. If your case is in state court, the applicable standard depends on where you’re litigating.