I’ve posted many times that the two ADs who positively gushed about our son after his interviews were his two rejections. Others will also tell you to read nothing into those comments as AOs see more terrific kids then they can admit, enjoy engaging with kids, and want to encourage all to complete their applications.
There is no list of questions to prep for as the best interviews are organic and can cover any ground the interviewer and student wander into based on interest. An interview might start with “tell me about yourself” or “why are you considering boarding school,” but those are only leader questions intended to draw the applicant out into more interesting territory. They are also the easiest to answer, so make sure your child can genuinely answer why BS and why that particular school but, beyond that, the interview should just be an easy conversation. AOs are not trying to trip kids up with questions. They just want to see that the student is enthusiastic and ready for the boarding (or day) experience at their school and can hold their own conversing with an adult.