Close Relationship with PortCo CEO

I sense you have some optimism that these conversations may lead to a full-time associate role at the fund.  I would say that that optimism is well placed -- you wouldn't get time with two partners and a director if not.

That said, you still need to treat this as an informational interview because you may come off presumptuous if you don't. And at the same time you need to be prepared as if this is a full interview because they might launch into that direction. 

In short, I think this intro you got is a double-edged sword.  You got a bit of an inside track but you actually have to double up on your preparation.

In any case, I would recommend you take charge of the conversation and let them direct you into a more traditional interview questioning if they like.  Below is a structure I would suggest

1. Build Rapport - "Hi, how's your day going? Are you in [HQ city] today or traveling?" This is not about becoming best buddies.  Just some light, small-talk (bland as it may be) to establish that you're a human.  No more than a minute or a few sentences.
2. Establish Intent & Structure - "Great - well thank you for taking the time to speak with me.  I'm looking to learn more about [XYZ fund / get into private equity, etc] which is why I asked [PortCo CEO] for the introduction. What I thought would be helpful would be for me to give you an overview of my background before I ask questions about your experience. Does that sound good to you?"
3. Deliver Your Story - This is the same thing you would respond to "Walk me through your resume".  Keep it between 1-2 minutes.  Try your best to tie in why your background has led you to say this is the kind of fund I want to join
4. Ask Questions - Try to come up with questions that will help you give better answers in a real interview setting. For example, if they say they work in lean deal teams, great now if you're in a full on interview, you can say I work at a MM bank where deal teams are lean and what I love about it is I get to step up on responsibility.  I'm interested in this PE firm because I know you are similar.  The other tact is to ask questions where they will get to brag (e.g., about a recent successful exit)
5. Close it Out - "This has been super informative. I think [XYZ firm investment approach / culture / etc] is really interesting and is exactly what I'm looking for.  I'd love to keep in touch and if there's an opening to join the firm, I'd love to be considered."

The tricky thing is if they do decide to actually interview you, you will need to also be on top of your game.  So a bit tough having to prep both ways but it's definitely a leg up to get an intro like this.