My understanding is they are just looking to make sure you understand the banding process, and that you understand and are ready for the changes you will be making. The questions asked of me by the psych I went to were basically family history, medical history, he asked me to tell him a little about the procedure, and asked my reasons for wanting it. All very simple.
Now, I have read about some doing "testing", etc, but that wasnt my experience. Plus, I went to someone recommended by my lap band doc, and so he completely understood what he needed to do in terms of eval, and believe me, he didnt do any extra (and I dont mean that in a bad way!) My doctor even read the eval comments to me and also said "everyone passes", so I imagine you'd have to be fairly delusional about the procedure and expectations and not ready to tackle the changes to not pass. Now, that was just specific to my experience, I wouldnt be surprised if there are some bad experiences out there with the eval, and not passing, but I'd bet a lot of those would be down to the insurance company having "crazy" requirements etc.
I say don't stress, be honest, and knowledgeable and you should be fine!
Best of luck!
Tina