I imagine you have been in this situation: Your key witness is a good man. You know he is honest. In all your discussions he is down-to-earth. Likeable. Not a whiff of arrogance.
Jurors are the single most important part of a trial. They decide who wins and who loses. While it is easy to focus on the arguments and the presentation of evidence - all of which are important - it is vital to remember who is actually deciding the case. Arguments do not decide cases. Evidence does not decide cases. Jurors decide cases.