What are the costs of expert witnesses in a personal injury case?

Costs awardable under this subdivision include 'costs of the services of expert witnesses, who are not regular employees of any party, actually incurred and reasonably necessary in either, or both, preparation for trial or arbitration, or during trial or arbitration, of the case by the defendant.' [Citations.]" (Najah v. Scottsdale Ins. Co. (2014) 230 Cal.App.4th 125, 143 (Najah).)