If you have a complicated personal injury case, there is a high chance you will need an expert witness to expound on some things during the trial. For example, an expert witness can explain technical aspects of a case that an average person may not understand without help. However, there are also things an expert witness cannot do, such as the following.
Argue the Case
Your expert witness is there to shed light on a specific issue, but not to argue your case. The expert cannot tell the court what to do or argue for a specific outcome. You and your personal injury attorney have the responsibility of arguing your case.
Say you have an experienced auto mechanic appearing as your expert witness in an auto accident case. The expert can inform the court why and how a brake failure can lead to the kind of accident you experience. However, don't expect the expert to argue for your compensation because the accident wasn't your fault.
You may be tempted to ask your expert witness for advice on the case, but the expert cannot do that. Maybe you want advice on how to approach your case or advice on what to do to convince the court that the defendant is liable for your damages. Such advice is out of the expert's scope of involvement in your case.
Exceed Area of Expertise
Expert witnesses are typically chosen for specific subjects. For example, if you need someone to shed more light on the financial practices of a company, you may need an accountant or another person with a good knowledge and experience of corporate finance. The expert will stick to that subject matter for the duration of their testimony. A financial expert can't testify on a corporate legal issue since that is outside their area of expertise; this means you need an expert witness for each subject that requires expert testimony.
Link Payment to Result
An expert witness can help you win your case or increase your damages, but they can't guarantee you such a result. In fact, an expert witness cannot ask for a certain amount of money in exchange for a guaranteed result, and you should not make a similar promise. After all, the expert is there to shed more light on an issue, and not to argue your case (your lawyer is available for that).
Your lawyer will tell you whether or not you need an expert witness for your case. Work with your lawyer from the beginning of the case, and you are unlikely to go wrong.