Here's a sweet story about a Pixar fan who teared up at the original WALL-E trailer: Pixar Honors the Girl Who Cried at the 'WALL-E' Teaser. I like a few things about this story. The original video is simple but heartfelt, showing a genuine reaction to the trailer. The story of Pixar flying Courtney in for the wrap party is terrific. But the part I liked best was what Andrew Stanton said about her:

Six months ago, when the first trailer for WALL-E came out, we were only halfway done with the film, and we weren't exactly sure how we were going to get it done. We were exhausted. And then, one day, a movie showed up on YouTube showing a girl watching the trailer for WALL-E. And every time she watched it, she would cry on cue. When we saw that, we knew we were on the right track.

I've never made a motion picture, but I know what it is like to fix onto one particular customer and use them as a proxy for all of them. Making a movie I imagine can become pretty removed from the simple question of how people will react to the film. When the Pixar people saw Courtney's strong reaction to WALL-E's character design, it was a huge boost for them. They had heard from their customer, and the response was positive. What more encouragement do you need?