Who is considered competent to be a lay or expert witness? If you are interviewing a potential witness during the course of an investigation, why is it important to determine whether a witness will be credible? What questions might you ask?

Under the Federal Rules of Evidence, any person is competent to be a lay witness as long as the individual has personal knowledge of the matter. Thus, only if a potential witness did not actually see, hear, or perceive the events in some way (because of a physical or mental disability, for example) will that person be judged incompetent to testify. Although state rules of evidence vary, most states also define competence for lay witnesses broadly. Expert witnesses are qualified only if they possess special knowledge, skill, experience, training, or education.
Because it is easy to establish competence for most witnesses, the primary issue is generally not whether a witness can testify but whether the testimony will be credible, or believable.
The parties to a lawsuit can attack the credibility of an opponent's witness and try to show that the witness is not telling the truth or is unreliable. In federal courts and most state courts, the credibility or reliability of a witness's testimony can be called into question by evidence, in the form of opinion or reputation, that points to the witness's character for truthfulness or untruthfulness. Thus, the paralegal investigating the case should inquire into any matters that tend to show whether the witness is honest. For example, does the witness abuse drugs or have a reputation in the community as a troublemaker or liar? Has the witness been convicted of a crime? If so, was it a felony, or did it involve any dishonesty or false statement? How long ago was the conviction?
Under the federal rules, a witness's credibility can be attacked by evidence of a conviction for a crime punishable by incarceration for over one year or a conviction for any crime involving dishonesty or false statements that occurred within the last ten years.