I remember feeling like my first semiotics class was eye opening. I had never considered that there could be an order to language or that that there was a science to understanding this order. Now, all this is a bit of an aside, but I bring it up because there is a parallel with usability testing. There is both an order to how people act and then a tandem act in which evaluators observe to make sense of what people do.
Video helps this latter act considerably. Without it, the evaluator will need to scribble notes and inevitably miss things. With the video, the tester has the audio, including all the textual responses, the gesture of the mouse, and the facial expressions. All of these tools are helpful in assessing usability.
The key is for the tester to create a framework where users feel comfortable testing the site, and sharing their ideas. Once that framework is in place, then one will find very useful information. But, without it, the user won’t feel comfortable sharing. A script helps the tester to be assured that they are saying the same thing each time. But, this script also helps the tester feel ready to put their user at ease.
Once that is done, one has the long task of many sense of the data. Often wading through all the information is almost as much fun as generating the data. Interpretation of evaluation data is the process of bringing into order disorder through noticing patterns. Once the patterns are clear, a good tester then develops a scheme to make sure that these patterns are obvious to anyone who reads the deliverable.