Jurors expect that professionals come to testify with opinions that support the side that hired them. Otherwise, why would they be testifying? However, they still need to be convinced that you truly believe the claims you are making. You have the important job of convincing the jury that you joined the case to investigate independently and evaluate the facts, that you done tests of your own choosing, and that you later reached the supportable and defensible positions you are presenting.
Initial, jurors have to be convinced that you deserve their confidence. You must have the qualifications, experience, or credentials required to do what you say you have done. Second, they need to believe that you fully understand the elements of the case and have taken the necessary and suitable steps and followed industry accepted methods. In short, you need to exude confidence that you have done these things, that you understand them, and that altogether, they led to your opinions.
People do not like to listen to boring witnesses. This has nothing to do with competence or qualifications; it has everything to do with effectiveness in court. A great expert witness, like a great teacher, sounds natural when he speaks, and appears personable and likable. He chooses words that are easy to understand, along with explanations that use interesting analogies or examples. He entertains with his voice and with any demonstrative exhibits he has created.
As an expert witness, be honest about what you do not know, and confident about what you do know.
Ideally, you want to have a human side that will make the jurors think back to a favorite teacher. A wonderful way to do this is to sound warm and enthusiastic about what you are presenting.
The bottom line is that your job is to persuade your listeners that your opinions and opinions are the right or the better ones. Some jurors will already have an opinion about the case by the time you testify. Others will have no opinion. In both cases, you will have to persuade those jurors that your opinions are accurate. Logic and facts presented by you will rarely be enough. reliable persuasion is essential. No small chore.