I would like to start off by saying some parts were a little rocky, but I've been on plenty of interviews and I've experienced worst first impressions. The interviewing process is quite generic, initial phone screening, coding challenge, and finally a technical and behavioral interview.
Initial phone screening was to determine if you were human.
The coding challenge was probably my favorite part. You're given a pretty straight forward app to build with a time frame of 48 hours (though there isn't really a hard limit to it, they understand everyone has a life). They give specifications and pictures as to how the application should be built, along with a PDF of Bottle Rocket's coding conventions. The code challenge tests your ability to implement core iOS functionalities without the use of CocoaPods. You are also encouraged to add your own "spice" to the application to where you see fit. The entire challenge however seems more to test how clean your code is and how you architecture it. Do not hesitate to ask questions.
After they review your code challenge, you will get scheduled for a 30 minute technical and a 30 minute behavioral interview. The technical interview was very easy going, they test basic knowledge to insure you are somewhat competent. Be sure to ASK QUESTIONS, they are absolutely willing to help, and even if you leave without an offer you'll probably leave with something to takeaway to your next interview. As for the behavioral interview, though perhaps won't be difficult for you, may be the make it or break it part of the entire process. Bottle Rocket seems to have a huge emphasis on culture fit, and if you don't fit it, they probably won't take you in no matter how much of a genius you are.