If the law has made you a witness, remain a man of science. You have no victim to avenge, no guilty or innocent person to convict or save — you must bear testimony within the limits of science.

- Dr. P.C.H. Brouardel, 19th-century French medico-legalist

The expert should keep in mind, at all times during testimony, that the expert is present as a witness to educate the fact finder (the jury or, in the case of a bench trial, the judge). Demeanor should be the same toward both attorneys, regardless of which attorney is asking the questions.

When answering questions, the expert's responses should be delivered to the fact finder, even if this means physically shifting your body or attention slightly to face the fact finder while answering.

During direct examination, most retaining attorneys will stand near the jury box, if possible, to facilitate this process. During cross-examination, a savvy opposing attorney will attempt to direct the witness's attention away from the fact finder by standing across the room.

In this cross-examination situation, it is acceptable, and perhaps even advisable, to listen carefully to the question while facing opposing counsel, and then turn with intention and answer directly to the jury or judge.

At no time during questioning should the expert ever become defensive, argumentative, or arrogant in answers to questions or in exchanges with opposing attorneys. The expert must remember that the purpose is to educate the court or jury on a subject about which the expert possesses specific expertise. The expert should never have a personal stake in the outcome of the case.

Testimony should always be based solely on what the specific analysis and test results can support. If the expert does not know the answer to a question, he should never try to guess or attempt to provide an answer beyond the expert's expertise or the scope of the expert's assignment and testing.

"I don't know" is a perfectly acceptable answer to any question, especially if the attorney is attempting to lead the expert outside his area of expertise.

Why would an attorney attempt to lead a witness?  The attorney may want to discredit the expert's credibility, and therefore the testimony, in order to win the attorney's case. The expert must remember that the courtroom differs from the laboratory. In court, the expert's duty is to express a scientific opinion that is supported by evidence, whereas the attorney's role is to serve as a legal advocate for the client and the case.

The expert should be aware that, if allowed to stray beyond the area of expertise by attempting to answer questions outside his qualifications, the court may allow the attorney some latitude to continue the line of questioning, to the eventual detriment of the expert's testimony.

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