Testimony may come from an eyewitness or from a non-eyewitness.
Eyewitness ~
An eyewitness is someone who experienced all or part of the event.
Non-eyewitness ~
A non-eyewitness is someone who did not experience any of the event but is in some way able to shed light on what may have happened.
A non-eyewitness can be someone who is connected to the event through a person, place or time, such as a grandmother who provides an alibi for a grandchild accused of aggravated battery or attempted murder when the government has no eyewitness to place the accused at the scene.
A non-eyewitness can also be someone who is in no way connected to the event but who helps to decipher facts relevant to the event, such as a crime scene investigator, an accident reconstruction expert, or in white-collar fraud or theft cases, a forensic accountant. A defense attorney should do everything legally and ethically possible to discredit this type of witness and this type of evidence. A jury often puts great weight on such testimony and evidence. If an expert or scientific/forensic evidence can be discredited, it is very difficult for a prosecutor to rebuild the expert or evidence in the eyes of the jury. A medical expert, or any other type of expert, should be challenged whenever it is reasonable to do so. The credibility of an expert witness and the reliability of scientific evidence can be challenged in different ways. If possible, a defense attorney should:
(A.) Present equally credible and reliable expert testimony or scientific evidence that demonstrates a contrary conclusion. This contrary evidence may bolster the defense theory, or at least neutralize the prosecution's position, and create reasonable doubt.
(B.) Critically scrutinize the quality of the expert's work. Experts should be challenged on the basis of their opinions; i.e. the raw data or the method(s) used to conduct their work.
(C.) Attack the collection or chain of custody of physical forensics to show possible contamination of the evidence as it was collected or transported between locations and among people before it reached its ultimate destination in the courtroom. Contaminated evidence should be excluded by the judge; if it is not excluded, it should be highly scrutinized by the jury based on the prompting of the defense attorney. Technicians who conduct routine procedures with sensitive equipment may sometimes make a mistake if they are rushed or tired. It is important for a defense attorney to check that scientific and medical or lab protocols were followed during the handling of the evidence. One type of case where this scrutiny is important is a D.U.I./D.W.I. case. The use of the breath analyzer machine is a critical component in this type of case. If a breath result can be excluded, or be shown to be unreliable, it could be the difference between a guilty or a not-guilty verdict in the minds of the jurors.