I had my first contact with Riot on GDC 2012, after giving it some thinking an cleaning my resume/cover letter I applied early on 2013.
Even though I had an inside 'connection', my application got lost in the stack, so I had to 'hunt down' through LinkedIn a couple of recruiters so I could get my first interview. Keep in mind Riot is a somewhat small (<1000) and seriously popular so the number of applicants versus the number of people that can review is seriously skewed. Also, a little comment about hunting recruiters, I don't think it's okay just to send 'I want to work for Riot', I had a position in mind and a few points of 'why' I would be a good fit, I think that helped me a lot.
After talking with two recruiters (one on the phone, one on person at GDC) the real interviews started, I had 2 phone calls that were mainly centered on my resume/experience and stuff I liked (technology, languages, personal projects) there were almost no 'hard' technical questions in the phone calls (unless you consider comparing a compiled vs an interpreted language a 'hard' question).
After the phone interviews I got 'green lighted' for an On-Site, it took a little time to get the date (around 3 weeks after 2nd phone call), again this was because there are more applicants than interviewers and Riot has a complex on-site paradigm (this I learned after going) that takes on a lot of people's time.
They payed for my flight, taxis and hotel for the on site and they were pretty accommodating (dates, hours). Once there it was a REALLY long interview day (got there 10 am got out ~6.30, non-stop interviews, so drink a coffee before or something). Interviews were pretty much 2 on 1, one of them doing most of the talking. There were management style interviews, experience interviews and full-on technical/algorithms interviews (be prepared to answer "Where do you see yourself in 5 years?" all the way to "How would you design a stack structure that performs well on memory and run time?"), brainteasers appeared more than once, usually Math related so brush up on that areas if you need to.
Playing the game is really important, they will ask you some generic stuff ("Favorite champ", "Stuff we have done recently that you liked/disliked", "How would you change this or that"). On top of that I had a lunch with a couple of people, this was more like a casual conversation but keep in mind their input will still count. Another highlight is that they asked me to make a small presentation about 'something I was passionate about in game dev', I think it is really important for Riot that you are passionate about gaming (in general, not just LoL) and having side projects or college projects in this area that you can show is seriously important, if you plan to apply and have nothing, then you better start looking for an interesting project (mine was pretty new and I started it because I wanted to join the industry).
I have interviewed with several companies in the past (Google, Facebook, Zynga, EA, Oracle, Intel and IBM) and I think Riot's is one of the most breadth intensive interviews, not the hardest thou, but they gouge on a great deal of diverse subjects, so be prepared. Riot also values culture/personality a lot (Way more than other start-ups I've seen), this is hard to fake and I wouldn't recommend faking it just to join a company if you'd later dislike the culture that surrounds you, so always be yourself and try to feel comfortable when speaking.
All of the interviewers were really nice people, I enjoyed my visit over there and learned a lot about a company that is growing like crazy, I encourage anyone reading this to apply if you find something you like there, but as a warning the company/culture might not be for everyone, keep that in mind.
