Interviews are a lot of back and forth. Shadowing is you sitting there watching the physician do his or her job, and maybe occasionally getting you involved. It's a much more passive experience.
Also, to further emphasize what Nick is saying... The interview is the culmination of your application. They judge you based on letters from people who have worked with you extensively, your grades, what you've done, how well you articulate your thoughts on paper, etc. In general, the interview isn't going to change the schools mind about you--unless you are wickedly charismatic or a social bore or do something egregious, like insult the secretary.