How can you make sure that the students watch the screencasts? I have developed several approaches to solve this problem. I often embed questions in the screencasts that students are responsible for answering for homework. I find that this reinforces the information that the student have learned. I often create online questionnaires  associated with each screencast containing several questions that students must answer including a question asking them to explain which concepts from the screencast they are having trouble understanding. I can then preview these answers before class which allows me to understand the problems students are having before I enter the classroom.  It also ensures that the students have watched the screencasts.  I implement these questionnaires in Moodle, but they can easily be done in Google docs as well. This technique was pioneered by Eric Mazur, a physics professor at Harvard University who has been very involved in active-learning.