In a recent diminution of value action, my wife wanted to take The Appraisal of Real Estate, 12th Ed., with her to court "for reference." I asked her, "Do you want to be cross-examined on that entire book?" Well, she didn't of course. Left it home. Talk to the attorney first. See if there's anything s/he specifically wants you to bring.
Taking the work file may be a good idea if your report is based on data and analyses not included in the report, but make sure that it doesn't contain anything which wasn't provided to opposing counsel during discovery. Go over it with your client-attorney. If, however, your report is self-contained with respect to data and analyses, leave it home. Take with you only the information you'll need for defense of your opinion.
"I don't know" is a direct and responsive answer. Use it.
"Yes, but..." and "No, but..." answers should never be used. Instead use "While it may be true that (whatever he said), (place the "but" part here)." You'll get cut off right after the "but.." otherwise. Think before you answer. Example:
Q: "Mr. Appraiser, the report you prepared does not contain a cost approach. Isn't it true that the cost approach is one of the three recognized approaches to value?"
A: "Yes, that's true, but..."
Q: Thankyouverymuch, next question.
Better:
Q: Same
A: While the cost approach may provide a good indication of value for newer properties, and is absolutely required for special or unique properties, it loses strength when applied to properties as old as the subject.
If you've written articles for trade publications, try to review as many as you can before taking the stand to avoid making statements which conflict with what you have written.
If it's a jury trial, wear a jacket and tie. Wear a shirt with a comfortable collar. If it's a bench trial, it still may be a good idea, but ask your attorney about it. Different judges have different preferences. Get a haircut. Shine your shoes. Look professional.
Avoid attempts at humor. Stick to facts and your expert opinion based on them. In a BK hearing, I was asked "When did you buy the truck?" I answered, "On Black Tuesday, November 3, 1992--the day Bill Clinton got elected." The court reporter and clerk cracked up. Opposing counsel had to make a visible effort not to laugh out loud. The Judge was a Democrat. Bad choice. (For both of us.)