From RockPaperShotgun, an interview with Kim Swift of Valve on Portal:
KS: I think it’s the process. As far as screen-testing goes, it’s usually not until the end of the movie process. We started testing as soon as we got here. And I can understand how it would destroy a movie, because it kind of did that for Narbacular Drop. We didn’t start play-testing until really the last month of our development, and by that point it’s too late to do anything about it. It’s like, “Oh, well, that’s unfortunate.” I think it’s when you start to get user feedback that’s most important. Because we’re in an entertainment industry, and if people are not entertained, then we’re not doing our job.