The difference between a logline and a taglineA logline is a one (or occasionally two) sentence description that boils the script down to its essential dramatic narrative in as succinct a manner as possible.
A tagline is a piece of marketing copy designed to go on posters to sell the film
– In space no one can hear you scream (Alien)
Crucially, a logline contains all the elements necessary for the telling of a good story. It is written for industry professionals to show them that you can create a viable story for the script – a marketing hook alone won’t cut it.
One further note that you won’t like: A logline is the DNA of your script. If you can’t make the logline work, it’s probably becasue the story in your script doesn’t work. This is why some people suggest writing a logline for your idea before embarking on the script.
Not sure where to begin? These tips are going to help:
1. A logline must have the following–       the protagonist
–       their goal
–       the antagonist/antagonistic force
2. Don’t use a character nameIt has no intrinsic information and so is a useless word. Instead, tell us something about the character.
– A sous-chef
– An ex-superhero
3. Use an adjective to give a little depth to that characterThis is your chance to show some character. Beware of cliche, and also of the power of irony. It’s helpful if the characteristic you describe will have something to do with the plot.
– A mute sous-chef
– An alcoholic ex-superhero
4. Clearly and quickly present the protagonist’s main goalThis is what drives your story and it will drive your logline too. Make sure that the goal is present early in the script – if you don’t make good on your logline’s promise early enough the rest of the script won’t get read.
– A mute sous-chef wants to win the position of Head Chef at her boss’ new restaurant
– An alcoholic ex-superhero searches for his daughter
5. Describe the AntagonistThe antagonist should be described in a similar, but preferably shorter, manner than the hero. If the hero faces a more general antagonistic force then make it clear that they are battling something, not just life’s bumps and buffets.
– A mute sous-chef wants must fight off an ambitious rival to win the position of Head Chef at her boss’s new restaurant.
– An alcoholic ex-superhero searches for his daughter after she is kidnapped by his dementing, jealous former sidekick.
6. Make sure your protagonist is pro-activeHe or she should drive the story and do so vigorously. Good loglines will show the action of the story, the narrative momentum that carries you through the script. In some cases the protagonist will be reactive, but note, this is not the same as passive.
7. If you can, include stakes and/or a ticking time-bombThese are very useful narrative devices that add urgency tou your script. If they fit in easily, include them in your logline.
– To save his reputation a secretly gay frat-boy must sleep with 15 women by the end-of-semester party.
8. SetupSome scripts operate in a world with different rules to our own and require a brief setup to explain them, e.g. most science-fiction stories. Others have a protagonist whose personal or psychological history is crucial to the story and needs to be explained. Again, be brief.
– In a world where all children are grown in vats…
– Driven to a mental breakdown by an accident at work, an aquarium manager…
9. About the endingDo not reveal the script’s supercool twist ending, even if it is the next The Usual Suspects. The story, and thus the logline, should be good enough to hold up by itself; a surprise ending should be a lovely bonus found when reading the script. N.B. This all changes when you get to writing your treatment.
10. Don’t tell the story, sell the storyCreate a desire to see the script as well as telling them what’s in it. Loglines are like poetry, every word counts. Tinker, test, and tinker some more.
BonusIf you can’t write a decent logline of your idea before embarking on the script, then maybe reconsider writing that thing. If it’s unfocused and muddled at the loglines stage, it’s not going to get any better as you write.