Mails and meetings, about what I have done before and some new work the professor is going to do. See if my skills match these requests and whether I am interested in his field of work

Hiring involved submitting of CV on intranet of school (crimson careers), and if interests match then professor sends you an invitation for interview. I think most of those who receive the invitation end up receiving an offer and accepting the job. The interview is very basic, it is just a gauge of the interest of the research assistant and also to see if the student has basic skills for the jobs.