One coincidence. No more.

Today, my friend and fellow screenwriter William C. Martell raised a key issue in screenwriting:

"You are allowed one coincidence in a screenplay - the one that kicks off your story. After that, EVERYTHING is cause and effect."

If your screenplay relies on coincidence to make ends meet, that coincidence needs to happen immediately before the story begins.

Why shouldn't it be Carl Denham who gets the map to Skull Island. If it wasn't Carl, the movie would be about some other guy who found the map and ran off in search of King Kong and fortunes untold.

In the middle of the film, even one coindicence ruins everything. For an example, let's look at the theatrical cut of Aliens, side by side with the director's cut.

In the theatrical cut, Ripley goes back to LV-426 because the company wanted to exploit her find from the 1st film. As the colony gets infested and overrun by aliens, the parent corporation realizes how badly they need Ripley. After all, she's the only one who knows what they're up against. When she meets the sole survivor, it's a little girl. That survivor could have been anyone. Because making her a helpless little girl doesn't make life any easier for Ripley it's not a coincidence. It's not useful.

In the director's cut, that little girl is the daughter of the man who first encountered the alien life form. REALLLY?!!! The first man to get impregnated and the last woman alive are related by blood? This family of rock miners is the Alpha and Omega of LV-426? What is this? Why is destiny screwing around with my marine assault survival horror action movie?

Then, we have the backstory of Ripley's daughter. You mean to tell me that Ripley meets this special chosen girl right when she's wrestling with the death of her own child? OH, PLEASE!!! JUST KILL SOME DAMN ALIENS, WILL YOU?!!

Funny enough, the theatrical cut loses none of its dramatic impact for doing away with these supposedly character-building details. By letting the characters feel more real, more happenstance, the theatrical cut allows us to believe and invest in them. Suddenly, running out of ammo seems pretty dire. Suddenly, Ripley's last-ditch effort to save Newt is one of the most heroic gestures ever captured on film.

Unless you sneak it in right before the opening scene, keeping coincidence away from your story will only make it better.