DoctorJ, I’m not your lawyer, you’re not my client, this isn’t legal advice and I’m not licensed in your jurisdiction, so read on at your own peril.
That said, are you being deposed as an expert witness or a percipient witness? I assumed from your post that you were being deposed as a percipient witness (i.e., a witness who has first hand knowledge of something, as opposed to an expert witness, who gives an opinion based on records, etc.), perhaps because you treated the plaintiff? If so, rules are slightly different for percipient witnesses than they are for expert witnesses.
Many states have laws that permit lawyers to pay the usual and customary hourly fee to certain professionals when being deposed (treating physicians, mainly), because otherwise they would never show up. If that’s the case, you can tell the lawyer who arranged your deposition that you’d like to be paid for the time you spent reviewing the file, your travel time and your testimony. If he tells you “sorry, the law says I pay you for your testimony only,” you tried.
As for the deposition itself, Ragiel has it spot on. The first rule is always tell the truth, no matter what. Listen to the question; don’t try to figure out what the question is getting at, but think about what the question is actually asking. Answer only what is asked, even if that seems really stupid. The classic example is “Do you have the time?” The answer is either yes or no. It is not, “It’s 7:30.” There are very good reasons for this rule, but this isn’t really the place to go into them.
Also, during the deposition, you’re Switzerland. You don’t have a dog in the fight; you’re just there to tell the truth. So if the lawyers start swearing at each other (or you), stay calm, stay focused and don’t get into the pit with them.
But bottom line, as long as you come out of it having told the truth, you’re in good shape. Good luck!