I applied online. The process took 5 days. I interviewed at Harvard University (Cambridge, MA) in Dec 2013
Interview
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.
I applied in-person. The process took 1 week. I interviewed at Harvard University (Boston, MA) in Sept 2017
Interview
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