My brother and I want to work on a screenplay together and are looking for a true real-time collaboration software that is also secure. Plotbot shows edits by all parties in real-time, but you have to trust Plotbot to safeguard your script as it lives on their server. I have tried Celtx but it wasn't true real-time. In fact, we never could figure out what series of steps finally synced the scripts. It made me nervous that text would be lost.
Has anyone had a good experience working collaboratively on a screenplay and would like to share what tools they used?
Edit: I know tools like Google Docs exist and while it has good collaboration tools, it doesn't have screenwriting-specific tools like formatting, cards, timelines etc. I am not looking for collaborative writing tools but rather collaborative screenwriting tools.