The idea for a movie came about from the success of Kingdom Hearts: Final Mix, in which the secret ending depicts a CGI action sequence of two people fighting Heartless. Director Tetsuya Nomura had the idea for a full movie with "lots of cool fight scenes", and thought Final Fantasy VII would be a good source material for it. The development team thought the ending movie for Final Mix was the best they could do, but Nomura was certain that with better technology they could surpass it. When Final Fantasy X-2 became a success the project came closer to realization. Nomura claims the merger between Square and Enix was the event that sealed the creation of Final Fantasy VII: Advent Children and got the project rolling. In spring of 2003 Final Fantasy X-2 was released, Square Enix was formed, and Final Fantasy VII: Advent Children was given the green light to start production.
The original script was only 20 minutes long and involved an unspecified woman giving a message to a child and asking her to deliver it to Cloud. Through a relay system between children, Cloud receives the message and at the end it is revealed who the messenger is. The script kept expanding due to a combination of fan speculation, the anticipation of a long-awaited sequel to Final Fantasy VII, and new ideas being worked in. Once initial reactions to sneak peeks and trailers were positive, the short video became a full movie. Yoshinori Kitase and Kazushige Nojima decided to bring Nomura in as director who began writing down key words for scenes as a foundation for a storyline. Some of the phrases he wrote down would be used for ads and trailers. For example, one of Nomura's ideas was for a child to narrate and explain events they could not truly understand, which developed into Marlene narrating the film's opening sequence which recaps the events from Final Fantasy VII. 2b1af7f3a8