I have had some interviews where I sat down and the entire time the professor grilled me on my research interests, future goals, and previous research. On the other hand, I've had interviewers that never even mentioned education, and just chatted with me about baseball and line dancing. (Seriously).
The point of the interview is to give the school a chance to sit you down and see what you're like as a person. If you're there, they obviously like you on paper and they are interested- but do they like you as a person? You could be a genius, but a jerk. Likewise you can have a rough application, but be a hard-working, charming individual.
Honestly, the interview depends on who you get- it's totally up to chance. You can believe your interview went poorly and end up acing it, or vice versa. The best you can do (in my opinion) is to smile, answer honestly, and try your best. If they like you, they like you. If not, then that's their problem