Graduate Research Assistant is a position which is offered by Professor to help them with their research or if you go to him with proposal for your thesis or doctorate research.
Interview preparation should be not done like exams it's basically you are trying to convince person that you are the right person for the position and that doesn't mean you need to give all the right answers to questions.
First thing is you have to find out what project you are going to work on and what are the requirements for that? - > You can talk with students which are currently working on that project or under that professor. This will get you basic idea on professor's research or projects going on in that lab.
Below are the things professor expects from you:
You should have taken classes related to position you are going to interviewed or you should have related course work, this two will help him decide that you are the right candidate for the position.
If you don't have prior experience related to position then also it's fine but then you have to check whether are you really interested in that position or not? If you are interested in that domain then check on online courses related to that domain and try to work on few projects.
If you don't have prior knowledge about project which you are going to work on then you should check professor's website and get more knowledge on what's his interest area and what he is currently working on, this will help you prepare about interview.