The case specifics may provide an opportunity to impeach a witness. This usually arises when a witness (expert or lay witness) has a financial interest in an aspect of the case or in the outcome. The financial interest may go beyond the obvious financial benefit of being retained in the case or being paid to travel first class to the trial. Financial benefit may even go to the opportunity to be retained in other similar cases.
Evidence Code §722(b) (pdf) makes admissible income earned by forensic experts on the specific case being tried. The expert’s annual income, or percentage of income, earned over the years doing forensic work is also relevant and admissible to question the credibility of the particular expert by showing bias. (Ev. C. §780(f) (pdf); CACI 5003) In the recurring instance where a retained expert will not produce documents in deposition that evidence past and present income earned in testifying, counsel should refer to Stony Brook I Homeowner’s Assoc. v. Sup. Ct. (2000) 84 Cal.App. 4th 691 (pdf). While Stony Brook does not permit unfettered access to an expert’s potential bias, it does support discovery into numerical estimates of defense and plaintiff-related work (e.g., including exams, reports and deposition and court testimony) and a numerical estimate of the amount of income generated from that defense and plaintiff-related litigation. In Stony Brook, the Appellate Court commanded the doctor to produce such records for the previous three years even if it took a third party to assist in compiling the information. Id. at pp. 698-700.
The final tactical decision in preparing to impeach a witness is when to do it. The witness will be on the defensive and likely after you have successfully attacked credibility, so consider eliciting facts supporting your case beforehand. However, because impeachment is such a powerful tool, you may want to consider it early on in cross-examination to place the witness into your control.