Giving your characters the right job can help your novel in three ways:
it helps with characterization
it lends credibility you, the author
it suggests ideas for plot
In medical novels, cowboy-themed novels, and detective novels, the job is the novel. If Miss Marple were not an amateur sleuth, there would be no story, for example. But you can still use your character’s job to characterize him/her, because the why of someone’s job is is more telling than the what. Is your character driving a bus because he likes it, or because it was the best he could get? Does your heroine work with abused children because her mother expected it, or because she’s trying to hear hidden wounds from her own childhood? Is your character’s job a daily trial, or the realization of a lifelong dream? A salesman who rushes to his first appointment full of excitement is different from one who loathes the very thought of another sales call. Regardless of the job, is your character good at it ( even if she hates it), does she care? Showing your readers the answers to these questions can reveal more about character than merely telling them she is a sales clerk or an accountant.
Giving your characters specific jobs also helps place them into a socio-economic level and can telegraph to your reader information about their backgrounds. Readers will make very different assumptions about a professor at Harvard Law than about a school custodian. But you can also surprise your readers by playing against type. Suppose the custodian speaks three languages and owns a classic automobile? Readers will turn the pages to discover why he chose to work as a school custodian. But you will have to work harder to show readers why he made that choice.
When choosing your characters’ jobs, consider their world view,, their self image, their natural abilities and their social class. To preserve your credibility, choose jobs that you know a lot about, or that you can learn about. Include enough detail to make the story seem real.
If you are working on a novel now, what jobs have you given your characters? How do those jobs inform your story?