The company that hired me literally had a stack of resumes waiting to be read, yet they asked to interview without me even directly applying. Being that the company already knows you qualify enough for an interview, personality is going to get you the job, hands down. I wouldn't stress too much since you are flight instructing and you should know the basic knowledge stuff already, study what the above post suggested. Since the interview is mainly a personality check, being humble will go a long way. It's your goal to show them why they want you over any other person, that means holding the door open for whoever you are with, showing interest in the little things, and dressing for the part. They are mainly wanting to see if you are willing to work hard with a smile on your face, have a solid grasp on safety, genuinely care about customer service, and are someone they think they can handle being around for hundreds upon hundreds of hours, in the air, and on the ground,
so be personable.
Other than the usual IFR knowledge, regs, and a basic grasp on jets in general, the following link almost mirrors they type of questions you will probably be asked and was pretty close to the kinds of things I was asked;
http://www.hiccups.com/interviewquestions.html
At my current job, my interview was given while sitting in the cabin of the plane (C560) by the captain, including a little Q & A about myself, questions about weather delays and how I would handle working with the passengers accordingly, what I would do if our passenger showed up extremely late and our duty time was about to expire, so on and so on. Since there is no one right answer, and you won't be able to expect all the questions, just go in with an open mind and enjoy. After the Q & A we went over basic systems in the cockpit, with the captain showing me the layout of the cockpit, which was pretty fun actually. A couple days later I was called in to fly right seat to complete the requirements of 61.57, so I could act as SIC for a flight with one of the two owners. A week after that flight I was given the job full time.
Flying in a jet for the first time be mindful of the required airspeeds when in bravo, charlie and delta airspace, those speeds will hit you
quick, if your not ready. Also keep in mind that you will be flying your TPA at 1500 ft agl vs 1000 ft agl. Also, make small power changes, as they will typically not be as small as you think. Don't be afraid to ask questions when they come up, such as for the proper power settings for your approach and so on. All the questions I've asked since I started, big or little, have gotten me a lot further ahead than if I hadn't asked at all. Take it upon yourself to learn from those who have been doing this for a long long time, and help them as much as you can when the time comes, which in return, will make them more willing to share the things they know with you.
Good luck