Boxes and Arrows has a very detailed tutorial by Alexa Andrzejewski, interaction designer at Adaptive Path, on creating prototypes in Flash.

To tackle the classic “how to prototype rich interactions” problem, I developed a process for translating static screen designs (from wireframes to visual comps) into interactive experiences using Flash. Requiring some fairly basic ActionScript knowledge, these prototypes proved to be a quick yet powerful way to bring interaction designs to life.

It's nice to see this kind of practical knowledge sharing happening on B&A. This is a nicely illustrated tutorial covering a very practical method you can start using today.