[Advice Needed] Afraid of Potential Exploding Offer

I’ve just went through a very tough interview process at a top BB - the hiring MD wants to call me tomorrow. I think that's a good sign, but am unsure... Any ideas?

Additionally, I’m also recruiting for a top PE shop. If given the chance, I’d perhaps take the PE offer, but am afraid that if I receive an offer from the BB, it may expire before I finish the process at the PE shop.

How should I respond to a (hopeful) offer given by the BB tomorrow? What would be a good strategy to let the MD know that I'm still super enthusiastic about the offer, but need some time to think? Any advice would be much appreciated.