Parents B2 Visa Help!

My parents will come to visit me again in the US within 6 months after they left US last time. I wonder if they will need to be interviewed again by the VO. I heard they only need to file some paperwork, not the interview. Also if it is true, when does the six months kick in, from last time they left US or from last time they got visa?

Thank you very much for your kind help!