Thanks for the suggestion of m-focus-mode and AF-C-button. Focussing is still a problem to talk about with fuji. I´ll try it soon.
The "real" challenge for video-clips is that you should have more than only "1" camera. Just a sample:
https://www.youtube.com/watch?v=jMj0FHWPPzQ
..and how it was made with final cut pro (sorry, but it´s only available in german)
http://dr-claus-sassenberg.com/?p=3801
With this in mind, it´s better to have 3 or more different cameras at the same time on the spot than only one - even if it´s the best.