At the very least wear dress pants, a collared shirt and tie....suit jacket optional... Can never go wrong over dressed....
After thousands of hours in corporate jets and the regionals, I interviewed at a small medivac job... First thing they noticed was the professional attire I had.. They went so far as mention one interviewee who wore jeans and a leather jacket... It didn't work out for that guy...
Never assume because it's a small company/ operation that they don't care how you look or represent yourself.
Depending on their experience, the CP will know what you've been doing the last few years. He'll know what the demands are on a regional pilot etc... They may ask you about your background, any problems at current job, any disciplinary action, why you want to go corporate, can you handle taking care of passengers. May ask you salary demands, so have an idea what you are looking to get. May also ask about having a. Problem with being on call etc... Just because we fly airplanes, and it is a different job than most, it's still an interview for a job. Treat it as such