Make All Characters Matter

I just watched the movie The Late Quartet. It's the story of a string quartet who, after 25 straight years of playing, finally have to deal with a whole lot of changes. It was a solid film. As with any story, I had some problems with it. One of them was an ancillary character who existed basically to ruin one of the characters' life.

**Spoiler Warning**

In one scene early on, the 2nd Violin player is jogging in the park with his jogging buddy (who is an abnormally attractive woman) and they're talking music. At one point, the guy explains that 2nd violin is an important part because it is what connects and blends the other three sounds and parts together. The woman then ignores everything he said and says that he should play first violin (all while stretching and bending over in front of him).

The violin player then brings up the subject of playing 1st Violin sometimes, which brings lots of hostility up and he has a fight with his wife (who plays viola in the quartet). In his anger, he sleeps with the jogging buddy and his wife finds out. This results in more than half of the problems that happen in this story.

But here's the thing: that jogging buddy never shows up for the rest of the movie. All she does is poison his mind and ruin his marriage. If she wasn't there everyone's life would be unequivocally better.

I really hate that. I hate useless characters. Or maybe I should call them utility characters. They only exist to make the story do what you want. Sure, you could argue that life can be random like that, but it just reeks of convenience here.

Make all of your characters matter. If somebody is significant in the plot, they should be critical. The world or story should be completely different without them. If taking out a character either makes everything better, makes everything worse, or brings the story to a complete halt, then that character doesn't matter enough, but should.