The interview process with a professor for a research position will vary by professor. For mine, it was pretty standard and moved fairly quickly. I applied through the website, and the professor reached out to schedule a brief phone call. Then, I went on campus to do an in-person interview a week later. That interview was about an hour long, and they asked me generally about my research experience and how it could relate to their research. They offered me the position on the spot, and I communicated my acceptance via email later on.