One way to add that mysterious quality of emotional layers or 'depth' to a plot is to have the hero's emotional journey echoed in a subplot. Alan Ball, the screenwriter of 'American Beauty,' does this masterfully.
This can be seen in how Wes Bently's (the intense young man in the film) plotline echoes Kevin Spacey's (and sometimes, vice versa).
First, there are some obvious parallels between the two men:
1. Both Bently and Spacey get fired by telling off their bosses.
2. Spacey and Bently are out of communication with those around them. Spacey lives in a sexual fantasy; Bently lives through his video camera.
3. Both take no responsibility for those around them. Spacey is content to let his family fall apart while he pursues Mena Suvari; Bently sees nothing wrong with the irresponsibility of selling drugs.
But Bently's storyline echoes Spacey's in a more important way as well. Kevin Spacey goes on an emotional journey. He starts in apathy, a vacuous wage slave. From this low point, he grows toward freedom and transcendence. His progress is mirrored in the character of youthful Wes Bently. With his poetic take on life, Bently seems, at first, to have achieved a kind of transcendence. However, we soon learn it's mostly a hollow dream.
1. He only videotapes people, avoiding personal contact.
2. He lets his father (Chris Cooper) beat him up.
3. He's fascinated by death and talks about it all the time.
4. He sells drugs.
Are these the actions of someone who has transcended and achieved a kind of enlightenment?
No. Wes Bently, like Kevin Spacey, is mired in apathy. Kevin Spacey gradually becomes more engaged in life, rising up from apathy to anger, embracing an almost teenage kind of rebellion. He gets a job in a fast food joint, like a teenager, and buys the car of his teenage dreams.
At the end, though, he does achieve true transcendence. Caring about someone else for a change, he turns down the chance to sleep with nubile Mena Suvari. It's his first step toward a transcendent perspective.
And from there, before and after his death, he quickly achieves true wisdom. He even speaks some of the same poetic words spoken earlier in the film by Wes Bently to reinforce the parallel between the two men.
Wes Bently, by trying to be transcendent at the start of the film, had skipped the all-important middle step of anger. So, at the end, he finally achieves anger and stands up to his father, leaving him forever. While he hasn't yet achieved the real transcendence which he fantasizes about, we feel there's a good chance he'll make it.
The bottom line here is that the two emotional plotlines roughly mirror each other, and that's a great technique for adding depth to your script. There are several other ways to use subplots to great advantage, but that's the subject for another article.