An ethical expert should not be offended by your questions or screening. For instance, someone who earned his degree many years ago in a broad field such as Electrical Engineering may or may not be familiar with the sophisticated electronic equipment your client manufactured last year. Although there are always some complicated, groundbreaking cases that will require extensive research, most experts do not need an unreasonable amount of time to get up to speed. You should not have to wonder whether you are paying someone to learn on the job. An expert must be free from conflict or prior obligation to any party to the dispute.
Objectivity is paramount.
While you hope your position and strategy will be supported, it is an expert's job to advise you of both the technical strengths and weaknesses of your case. And your expert has to feel free to tell you if a case has little or no merit. You have to create an atmosphere where this is possible.