I've read that an interview is typically a good sign, but that you should still not read too much into it. Be confident, informed, and personable. I imagine if you prepare like you did for the GT interview, you'll be fine.
@"Paul Caint" Nope. Chicago tends to do fewer, bigger waves. So they interview a bunch of people over like 3-4 weeks and then admit them all at once a few weeks after that. I suspect I'll hear in another 1-3 weeks with the next wave.
@"Raphael Rivera" - thank you for this - it's very helpful. I'm sorry your interview didn't go well - do you mind me asking why? I wish you the best of luck in your application process!
I don't have any advice but I just wanted to say congratulations. Getting an interview is an accomplishment in itself and a good sign that you'll be admitted.
You can always contact Spivey consulting and get interview prep from them. It's pricey but if you really want to go to Harvard it would be worth it just to ease the nerves and know what you're walking into.