Designing Twitter Video is a really interesting look at how the twitter mobile teams prototyped and implemented their new video feature.

Who are you prototyping for? Prototypes are partly for me to get a sense for what something could feel like and validate the interactions of the concept. However, they are very much also for communicating intended designs with all stakeholders. I would often show early prototypes to my manager Brendan Donohoe for his thoughts along with others at design critiques, in addition to emailing screencasts to the video team.