Expert witnesses are hired to share their expert opinions of case data based on the scientific analysis of said data. They are experts in a specific field, such as a bank expert witness, and only present the facts of the case and the logical conclusion of those facts.

What They Do
Expert witnesses will be given instructions about a case. They will then review all the documents related to the case that fall within their area of expertise. For example, a banking expert may go over banking records and financial reports to determine if fraud, tax evasion or other financial crime has been committed or to determine whether assets are being reported correctly. The expert then provides a detailed analysis of the documents and formulates an opinion based on the data.
Their Opinions
These witnesses must provide truthful, impartial opinions. Their opinions must be based solely on the evidence they are given. They should be able to present the evidence and show the process they followed to develop their opinion. This must be a logical, scientific process. Therefore, if the data does not support your case claim, experts cannot twist the truth just because you pay them.
What They Won’t Do
Expert witnesses are not investigators. They do not typically provide outside evidence but analyze what they are given. However, they may identify missing or questionable data. They are not legal professionals, so they won’t typically give you advice on how to try the case. Also, their expertise is limited, so they won’t answer questions that are outside their fields. Although they can present facts to both parties, through depositions, they are not typically involved in negotiations.
Their Use
Experts are often used early in cases to resolve disputes without going to trial, but they are also used during trials if the case cannot be settled.
Any case that involves complicated data, concepts or processes may benefit from an expert witness. If you are trying a complicated case, consider hiring an expert witness.