Difficult but do-able, didn't ask anything that felt overwhelming. Good questions and friendly people, if you're willing to put in the time its a good place to land. If you get the chance, I would recommend.

The interviewer started with basic questions, for eg courses taken in the semester. Then i was asked basic conceptual questions of electrical engineering, as basic as resistance, capacitance, magnetic and electrical fields concepts. I was asked to explain my past work projects and current study projects. Interview lasted 45 minutes and was an good overall experience for me.

There is no formal interview, but you need to get good grades from the professor you want to work with. He/she will give you a small amount of work and expect you do that. You must be eager to learn about his/her research field and show that you will be available to work for long period of time (a few semesters or years). If you have already had a publication at the same or related field, it is a big plus. However, this all highly depends on the professor, how many students he/she has and his/her funding.