Retain your expert witness as early as possible. The first advantage is that the expert can help you decide if the case is questionable or a no-merit case before you have spent time and money on it. If that determination is made, defense counsel will gain strength in their argument for summary judgment or a lower settlement.
The expert can also educate you to gain a working knowledge on an esoteric subject. For example, a weather and meteorology expert witness can supply books and articles for you to get an understanding of aviation meteorology, air dispersion, air visibility, and satellite imagery analysis, as well as other topics.