1. A method of indexing a stimulus comprising: measuring a plurality of facial expressions in an audience of the stimulus; interpreting measurements of the facial expressions using a processor, wherein the processor interpreting the measurements produces a result estimating a mood for a time period of the stimulus, and wherein interpreting measurements of the facial expressions comprises: classifying each of the facial expressions as corresponding to an emotion from a set of emotions; and statistically analyzing the respective emotions corresponding to the plurality of facial expressions to estimate the mood; generating an annotation of the stimulus, wherein the annotation is generated based on the mood estimated; repeating the measuring, interpreting, and generating steps to produce a plurality of the annotations that respectively correspond to different time periods of the stimulus; and indexing the annotations according to the respective moods, wherein the indexing provides a two-way linkage between the annotations and the time periods of the stimulus.