The first thing I tell my students is not to be too nervous about their college interview. Why? Because the interview can’t hurt you, it can only help you.
Alumni interviewers get very limited information about you because the admissions office want an unbiased opinion about your personality and interests. What these alumni are looking for is a chance to get to know you and your interests related to the particular university you're interviewing for.
It might not help you though. For example, if you’re nervous and give incomplete or difficult to follow answers, there won’t be much for the alum to write about.
When I was an alumni interviewer for Penn, I met a lot of amazing students. Ultimately, these students were accepted or rejected because of what was written and demonstrated on their application--not because of anything negative or positive that I said about our conversation.
After the interview, the alumnus/a will write a report detailing their impressions of you. As long as you don’t say anything disrespectful or inappropriate in the interview (this is why it’s helpful to have a mock interview ahead of time to double check what impression you give), this report won’t hurt you.
If you do 1-2 mock interviews, prepare your answers in advance, and get feedback, you have nothing to worry about. Colleges will use these interview reports in a very limited capacity.