In What are the qualifications to be an expert witness?, David Goodwyne writes:
b) Experience – Obviously the expert witness should be someone with the relevant practical experience in the field which he claims is his scope of expertise. A “forensic expert” who has never been inside a lab is not going to be of much help to you. In contrast, a foresic expert who has 20 years experience working with various governmental investigative bodies would, definitely have more of a presence.
(c) Demeanour – At the end of the day, the expert is trying to convince the judge or jury that their opinion is viable, accurate, and sound. As the judge and jury are all human beings, simple things like demeanour can also sway their mindset. Therefore you may want to avoid a pompous expert, or conversely, one who seems to lack conviction or confidence.