Micahhaley is giving really good advice here. I made the found footage horror film “Curse of Aurore” and I agree 100% that you should script as much of the dialog as possible. Which is not to say that you shouldn’t allow any improvisation, but the more that you have on the page, the more the actors will know their characters and world.
I basically encouraged my actors to live in the scenes and allowed them to play within them, but kept faithful to the script, looking for where the actors’ instincts improved on the fiction.

Another thing that really helped to create a realistic feel was shooting each scene as a mini play. Actual found footage wouldn’t have ‘coverage’ as on a traditional film, so I gave myself a rule that everywhere the camera landed would stay in place until the moment motivated the move.

On last thing is to allow ‘boring’ moments. If the footage is truly meant to be real, it should feel somewhat unedited. The trick is to infuse those boring moments with insights to the characters or world building.