Applying for research assistant (What can I speak about?)
I've been interviewing students recently and there are three questions I ask myself after talking to the student:


Motivation. Is this person independent and able to solve problems on their own?
Overlapping research interests. Is this person genuinely interested in the type of work that I'm doing?
Technical ability. Is this person going to be able to complete tasks without being hindered by a technical limitation?


It is difficult for me to accurately answer these questions from a short interview, but you can learn a lot from just talking to someone. My advice for you would be to figure out how you can honestly answer those three questions for the professor. Some things for you to brainstorm about: Have you read some of their papers? Have you worked on any class projects or side projects that may be relevant? What do you want out of the position? Do you have an interest in publishing or going on to a PhD?

Of course, there are other factors at play. Can the student communicate well? Will our personalities clash? But those are mostly out of your control.