Got interview to BCG General Associate position, how to best prepare?

Hey guys, as the title says above I just received an email from BCG for a first round interview completely out of the blue.

A little background: I applied for the position online like 2-3 months back, forgot about it, and within the time period already accepted an offer at a boutique IB where I start in 5 days, I also have work experience (role that i recently left) working as an IB intern for 6-7 months working on deals in EM which from my understanding is that MBB has been aggressively trying to dominate in (which i also think is a possible reason as to why I may have gotten selected in the first place at all). I'm also from a non-target school, graduated May 2018.

The interview for BCG is in 3 weeks.

My question is how do I best prepare for the interview with such a short window of time. My understanding is that people spend MONTHS prepping for these interviews (as they should, just like I spent months prepping for IB interviews myself to show I can do the job) My long term plans are to go into PE and I'm leaning in staying in Banking and doing the whole IB --> MBA-->PE path but I figured why not take the interview because it's not everyday where BC fucking G selects a non target kid for a consultant interview and also because consulting always seemed like an interesting career.

I've begun looking at case studies and I've had some experience in consulting-like thinking when working on deals since my director showed me to view businesses operationally and strategically which I used for my investment theses, however I feel vastly unprepared for this interview since I wasn't even expecting it to happen.

Any tips for case interviewing would be much appreciated.

Thank you all in advance. I hope I'm not sounding like too much of a dip shit when luck just throws an opportunity at me out of nowhere.