In the first ten or fifteen minutes of a movie, the audience is trying to determine the world of your story and the rules of that world. They are open to just about anything as long as it’s logically consistent. Are there giant monsters in this world? Okay. Do people break into song and dance in the middle of the street? Sure. Have mutations given people superpowers? Yep, I’m with you.
Then at some point the audience begins to feel they know what world they’re in. After this they will reject any new fantastical element you try to introduce. So it’s important early in your story to at least hint at any unusual things you want the audience to buy into.
For example, we suspend our disbelief about zombies in Zombieland (written by Rhett Reese & Paul Wernick) because we are told right up front that they exist - in fact it's in the title. But if zombies appeared in the middle of a movie like Little Miss Sunshine (written by Michael Arndt) it would seem ridiculous. It would even seem ridiculous in The Sixth Sense (written by M. Night Shyamalan) - we're told ghosts are real in that movie, not zombies.
Sometimes, however, we want to hold back a piece of information to create a surprise or twist, or just to avoid overloading the movie with exposition up front. In The Sixth Sense, Cole doesn’t actually deliver his famous line, “I see dead people,” until the middle of the movie. Ghosts aren’t explicitly discussed until that point. But we have to be prepared to accept them. So we get the creepy scene where all the drawers and cabinets in Cole’s kitchen mysteriously open. We’re being prepared that something supernatural is going on so that it’s believable when it’s finally revealed that ghosts are behind the strange happenings. You can get away with this as long as the additional element falls within the bounds of the kind of world you’ve set up.
In science fiction or fantasy stories, the world may be a believable representation of our world with one or two fantastical elements added such as in Back to the Future (written by Robert Zemeckis & Bob Gale) or Harry Potter (screenplay by Steve Kloves); or it could be a near future projection of our world such as in Children of Men (screenplay by Alfonso Cuaron & Timothy J. Sexton and David Arata and Mark Fergus & Hawk Ostby); or it could be a vision of our world projected far into the future that is both familiar and very different such as in Alien (story by Dan O'Bannon and Ronald Shusett, screenplay by O'Bannon); or it could be a complete alternate world such as in Star Wars (written by George Lucas) or The Lord of the Rings (screenplay by Fran Walsh & Philippa Boyens & Peter Jackson).
That decision will affect how much work you have to do to bring us into the world. In a world we’re familiar with you don’t need much set up. In The Terminator (written by James Cameron and Gale Anne Hurd) the writers don’t have to explain Sarah Connors’ lifestyle because it’s one we’re familiar with. We know about waitresses and single women getting stood up by their dates and nightclubs. What the writers have to do is hint to us that time travel and killer robots are going to be a part of this story. They do that by showing a naked Arnold Schwarzenegger appearing in a glowing ball of energy in the opening.
The world of The Lord of the Rings, however, is completely unfamiliar. The filmmakers have to explain how magic works in this world and the politics of hobbits, elves, dwarves and men. That’s a big reason why the films open with a long expository prologue. They need to teach us the unfamiliar history of this world.
You also have to show us the rules of the fantastical elements you introduce. The most important thing is that your story has an internal consistency and logic. You get to create the rules but then you must abide by them. You can't add to them or change them later unless you carefully plant the changes. Otherwise it will feel like “cheating” and the audience will reject your story.
The technology of Star Trek and of Alien are very specific – and not the same. Teleportation exists in Star Trek but not in Alien. And the conditions required for successful teleportation in Star Trek are clearly laid out. You need a teleportation room on one end and the range is limited. Also, the faster someone is moving the more difficult teleportation is. So the writer couldn't decide in the middle of the story that someone has a handheld teleportation device unless they set that up.
Musicals have a similar issue. There are two basic types of musicals. First we have those that are set in the real world where the music in the film comes from performance. This would include films like Ray (story by Taylor Hackford and James L. White, screenplay by White) and Almost Famous (written by Cameron Crowe).
Then there are musicals that exist in a world where people break into song on the street and bystanders instantly know and join in with choreographed dance numbers. These are movies like Singin' in the Rain (written by Adolph Green and Betty Comden) and Les Miserable (screenplay by William Nicholson & Alain Boubil and Claude-Michel Schonberg & Herbert Kretzmer). In this second type of musical we are operating in a fantasy world, which means we need to introduce the audience to the fantastical element early. If the first time someone breaks into song in public is an hour into the film (and it’s not a dream or a psychotic break), we won’t accept it.
So think carefully about the fantastical elements in your story and how you're going to introduce them. Make sure to get the word out to the audience before their suspension of disbelief hardens.
In other news, the book I wrote with producer Ken Aguado - "The Hollywood Pitching Bible" - is getting close to release. We just received the proof copy this week (and found an error on the cover that is being fixed). We also have a website: www.screenmasterbooks.com. Check it out!