Interpreting the script and making it into a video. This can involve planning locations, shots, pacing, acting styles and anything else which affects the feel of the video.
Overseeing the shooting and technical aspects, including lighting, sound, framing, etc.
Coaching actors and non professional talent and directing them towards the best and most believable performance
Coordinating the staff on set, directing the shooting timetable and ensuring that deadlines are met.
All of this takes knowledge and experience.