Applied online, got a phone screening with an entry-level marketing person a month after applying, got invited to interview on-site a month after the phone screen, got the results 4 days after the on-site interview.
The phone screen consisted of very typical questions - tell me about yourself, why Microsoft, and 4 behavioral questions about working in teams, solving conflicts, overcoming disagreements, etc.
The on-site interview consisted of 4 back-to-back 45 min interviews with various marketing leaders from all different levels, from entry-level marketing managers to senior marketing directors. You start off in a room with all the interview candidates for all the internship roles, and you get time to network with fellow interview candidates and a couple of full time employees. You then go through the 4 interviews in a row. Each interviewer was different, but most interviews were focused on behavioral questions. They asked a lot of questions about collaboration. I was also asked a lot about specific resume experiences. The marketing-related questions were fairly basic for the most part, but I did get some that threw me off. After the 4 interviews, you get treated to lunch and a brief campus tour, then you're free to go home. Microsoft pays for all your travel and gives you a generous budget for food, sightseeing, etc. Overall, the on-site interview experience was awesome!
Advice: read "Rise Above the Noise" by Lewis C. Lin. Read Microsoft's annual report and gain a deep understanding of their product line and where their products stand within the tech industry as a whole. Be educated on current events in the industry. Prepare a personal and unique answer for "Why Microsoft?" Start every marketing question by thinking about the consumer. Have examples prepared for every common behavioral question. Make sure your answers show that you align with Microsoft's culture and values. Emphasize teamwork and leadership abilities. Be conversational and don't sound like your answers are scripted.