GIMME GIMME GIMME
In story development there is a term: gimme. A gimme is a story beat that’s built on a coincidence. There are big gimmes and small gimmes. Typically, a well-constructed story has room for one big gimme.
Even then, the big gimme is usually found in the set-up, i.e. the coincidence that either creates the set-up, or gets our protagonist into the set-up. For example, in AVATAR our gimme is that Sully’s brother was fitted for a Na’vi avatar, so he’s the only person who can also inhabit it. In ALIEN our gimme is that Ripley’s ship the Nostromo just-so-happens to be the ship that is sent to the xenomorph planet. In GROUNDHOG’S DAY, Phil just-so-happens to be the guy who falls under this curse, and so on.
Beyond that one big gimme, we have to do the work of selling the story beats. Because when it comes to narrative, coincidence is the weakest of all construction materials. Using coincidence to build the story is exactly like putting up a house that’s made of balsa wood and paste glue; one stiff breeze knocks the whole thing down.
Coincidence comes with the lure of easy solutions. What if our protagonist just-so-happens to end up in the right place, and know the right person to get him to the next plot point? What if she just-so-happens to find the files she needs to solve the case, which the bad guys have been kind enough to leave out in the open?
Simply put, any story choice that leaves the audience free to think, “Boy, isn’t that convenient” is a gimme. The trouble with leaning on gimmes is it’s not just lazy, but obviously lazy. So obviously lazy that the audience can easily pick up on them. Once they notice one gimme, they start looking for more. And a script that’s willing to indulge in gimmes probably has more.
So instead of sitting back and engaging with the movie, the audience is now disengaged. They aren’t with the movie, they’re against the movie. They’re looking for plot holes, things to laugh at and make fun of. Watching the movie becomes a game of finding the gimmes. Then they come out of the theater and tell their friends and family about the movie they just saw… a movie that “makes no sense.” Because it’s constructed on gimmes and coincidence.
Here's an example of gimme vs. sold. In STAR WARS (the original) our gimme is that Uncle Owen buys these droids that just-so-happen to be on a mission to get to Obi-Wan. This gimme is necessary to the story because it gets our protagonist, Luke Skywalker, into the story.
Luke meets Han Solo and Chewbecca. Is this meeting a gimme? No. Because he and Obi-Wan go to Mos Eisley, looking for a ride. Mos Eisley is a “hive of scum and villainy.” The kind of place smugglers might hang out. Smugglers like Han and Chewie. Their presence in this location is sold, no gimme.