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So long as there are lawsuits, there will be opposing experts, with equally impressive credentials hired to express contrary opinions for the side that brings them to the dance. But what do you do when the your opponent discovers that, like most honest experts, yours concedes that there is an “on the other hand” side to their opinion?

Suppose your opponent decides that they like your expert’s testimony so much that they want to offer it in their case?

“Ladies and Gentlemen, don’t take my word for it. Take the words of the expert retained by the defendants themselves. The distinguished doctor whose credentials are so unassailable that they agreed to pay $1,000 an hour for his opinions. And he dutifully came up with the opinion they happily paid for; but then he conceded there is the ‘other hand.’ ” Choke.

Now, you know that under the Federal Rules of Civil Procedure you have no obligation, absent a showing of exceptional circumstances, to disclose the identity, much less the opinions, of experts retained in anticipation of litigation who are not expected to be called as witnesses. But what happens once you disclose the expert as a potential trial witness? Can you take it back? Can you stop your opponent from using your witness by saying that you no longer intend to call the witness?

Your case-management order requires that the parties simultaneously disclose the identities and general subjects of expertise of witnesses, with reports to follow 30 days later. So suppose you designate Dr. Who, but decide not to use him as a witness before filing his report? Can your opponent, Mr. What, serve interrogatories to find out what Who was going to say that got him de-listed? Or suppose you decide not to call Who after submitting his report but before his deposition? Can your co-defendant, Ms. Ida Know, nevertheless compel a deposition? Who’s on First?

Once a report is filed the expert belongs to either side. Actually, the answer is far less complicated or amusing than an Abbott & Costello sketch: You can change your mind and shield a witness from discovery after disclosure and before a report is tendered; but once an expert’s opinions are disclosed, they are fair game and can be used by either side. The mere revelation of the name of the witness doesn’t preclude a change, of course, because a disclosure under the Federal Rules of Civil Procedure requires both disclosure of the identity of the witness and an accompanied written report containing the opinions. But the report is the point of no return.

Prior to 2009, the prevailing view was that one could change one’s mind at any time. If a designating party withdrew its disclosed expert as a witness, the witness was magically reconstituted as a consulting expert immune from discovery. But Judge Frank Easterbrook of the U.S. Court of Appeals for the Seventh Circuit took the magic out of the equation when he held, “A witness identified as a testimonial expert is available to either side; such a person can’t be transformed after the report has been disclosed.”

Now if it happens to you, be sure to ask for an in limine ruling that no mention be made of who retained the expert. It won’t take all of the sting out, but it will help. However, if you forget to ask, don’t assume the appellate court will fix it, since courts find it is error — but not reversible error — to allow the opposing party who called the other side’s expert to elicit testimony about how the witness was retained.

So it’s a good idea to depose experts after all. I have written in prior columns about the calculus of taking an opposing expert’s deposition. I cautioned that you should think twice about doing it, since you are far more likely, by taking the deposition, to expand the permissible scope of the expert’s trial testimony than you are to learn something useful. This was not an original thought by me. I stole it from my friend Gregory Joseph, although I suspect he, in turn, had stolen it from Homer or some other classic.

But the possibility of opponents using adverse experts’ testimony raises a completely new issue. If you think there is any possibility that you may want to offer testimony from your opponent’s expert, then you may want to preserve the testimony in a deposition. The court may allow you to call the opposing expert, but how do you get him or her to trial? An expert is not a party opponent — the report is not an admission. Your opponent has no obligation to bring a nonparty expert to trial. If the expert is beyond the subpoena power, and you haven’t deposed the expert, your attempt to turn the tables by using the other side’s expert may get turned on you when the other side decides not to bring the witness to trial.

So on the one hand Who’s on first, and on the other hand, What’s on Second and Ida Know what to make of it. But don’t assume that you own your own expert as a witness.

Robert L. Byman is the president of the American College of Trial Lawyers and a litigation partner at Chicago-based Jenner & Block. He represents clients in complex ­commercial cases on issues ranging from patents and intellectual property to civil rights. He can be reached at