Bit of a tough one because CP's are all different, but at least know what an operations manual is and how to read one. Know where the current rules pertaining to charter are and how to look them up. Find out what the company does before the interview, what they operate and where they operate
As an aside, i recently interviewed two instructors, one didn't know what Part 61 was, the other had a copy on his Ipad. You can guess which one got the job. For charter I would expect the same with current (charter) rules.
I am sure others will add more, that's all I can think of currently.