What a firm means with the phrase "This is non-evaluative"
A number of firms have "non-evaluative" mechanisms in place to help candidates feel comfortable. McKinsey had their non-evaluative buddies, BCG has a non-evaluative coffee chat, and almost all firms have their "pre-interview dinner". Ultimately, the goal is to let the candidates be themselves and learn about the firm in a non-threatening environment. Below I discuss what the phrase non-evaluative really means.
I'm going to borrow a phrase from a friend of mine at BCG: "If you really think they're non-evaluative you're an idiot".
I can't comment on the accuracy of the statement, but I can say that I completely agree. The fact is, no matter how 'non-evaluative' an event might be, you are always being assessed on your personality and fit, and in fact informal situations are one of the best ways for a firm to get a sense of how you fit the firm culture. If you're rude to your McKinsey buddy, reschedule appointments, or make any sort of inappropriate comments, I'd be willing to bet that he or she would find a way to let the recruiter or other consultants know (even if it's not through formal channels).
I've personally heard comments from partners who took offense to candidates who were disinterested during a firm dinner, couldn't hold a conversation, or spent their time bbm'ing on their phone (pre-interview dinner is not the time to give your girlfriend a play-by-play of what's happening). This will push you from a "Yes" to a "Maybe" or from a "Maybe" to a "No".
Remember - firms spend tens of thousands of dollars on recruitment ever year, with costs from hosting information sessions to transporting consultants half across the world to the opportunity cost of borrowing consultants for interviewing (as opposed to doing billable work). Any additional data point they have to pick the best candidate is highly valued.
So at each of these events - be engaged, interesting, and please don't spent your time engrossed in your phone or staring at the walls. You are absolutely being judged for the way that you act, both positively and negatively.