There was a period in the 90s where I was writing a lot of science fiction screenplays because it was a popular genre. For the most part, a science fiction movie is really just an action movie that takes place in the future. Sure, there are exceptions like GATACA, but mostly you have TOTAL RECALL and JUDGE DREDD and TERMINATOR and I ROBOT and MINORITY REPORT. In a science fiction script you start with what one big thing makes the world different – Apes have taken over? Food shortage plus population boom so we all eat Soylent Green? The Zombie Apocalypse happened and now you are the Last Man On Earth? There are psychics who tell the police who to arrest before they commit the crime? There is one big change, which is tied to the theme (point) of your story.
–Bill Martell, screenwriter (and practical man)