Tips for lunch with client?

I've been on a project where I've been leading a lot of the workstreams as an Associate, and I'm going to lunch with my MD and the client corp. dev VP - just the 3 of us. I've had meals with clients before, but always with a larger team or a "lunch" that was actually a presentation. This will be the first time in a more informal context (without an agenda) in a much smaller group. Any tips on things to talk about? How to act? Etc.? I'm generally pretty socially anxious...