Since I'm a writer, I naturally think that all the writer-stuff is important in a work of fiction: the story, the plot, the setting, and so forth. When I gripe about movies that bother me, my complaints are usually about writer-stuff (and music; I pay attention to music).

So it's humbling to realize that one of the most popular characters in modern cinema came about because of a picture.

When George Lucas was writing the first Star Wars film (called Star Wars, none of that "A New Hope" nonsense), there was a secondary character in it called General Darth Vader. He wasn't the main villain — that was the icy Grand Moff Tarkin, commander of the mighty Death Star. He was a high-level henchman; what the indispensable Tropes Wiki refers to as "The Dragon."

Once Lucas started turning his script into a film, he hired the legendary concept artist Ralph McQuarrie to do character illustrations. McQuarrie read the script and noted that General Vader's first appearance comes when he leads a boarding party onto the Princess's spaceship. Boarding a ship in space implies that you're wearing a spacesuit, so McQuarrie came up with a really impressive design for the General's spacesuit, incorporating elements of Japanese armor, a skull-like facemask, and a billowy black cloak.

When George Lucas saw the picture he was blown away. He expanded the role of General Vader in the story, and came up with a neat bit of character background: the suit wasn't just a spacesuit, it was a life-support system that Vader had to wear all the time. The sound crew devised a very sinister breathing sound to go along with it, and got James Earl Jones to record Vader's ominous bass voice. To reflect his new importance, General Vader became Lord Vader, a Sith Lord (whatever that might be).

After the success of Star Wars, Lord Vader only grew in importance. In the first sequel we discover that he's the right-hand man of the Galactic Emperor himself, and that he's secretly the father of young Luke Skywalker, the hero of the series. In the final film, Luke goes all out to help his father redeem himself and overthrow the Emperor.

A decade later, when George Lucas returned to the universe he made, the "prequel trilogy" was all about the rise of young Anakin Skywalker, and how he turned into the iconic villain Darth Vader. The whole film series is about him now, even though he began as just a supporting character.

All this, from one picture by Ralph McQuarrie.