Streaming digital video is becoming increasingly widespread. How should video presentations be designed for web access? How is video accessed and used online? We examined detailed behavior patterns of more than 9000 users of a large corpus of professionally prepared presentations. We find that as many people are accessing the talks on demand as attend live, but online access patterns differ markedly from live attendance. People watch less overall and they utilize the ability to skip to different parts of a talk. In designing presentations that will be viewed later on demand, speakers should emphasize key points early in the talk and early within each slide, use slide titles that are meaningful outside the flow of the talk, and reveal as much structure as possible in the slide titles. The results also provide guidance for those developing tools for on-demand multimedia authoring and use.