When I conduct a writing workshop, one of the questions people frequent ask is about the importance of plot versus character. I tell them that it depends.
If you're writing a novel, or maybe even a series, you need to know your main characters very well. These imaginary friends and co-workers need a biography that's complete enough to flesh them out and show what makes them who they are. You need to understand their strengths, weaknesses, and the lines they won't cross well enough to know what they want enough to risk dying for it. If you write mysteries, you need to understand how your protagonist's mind works so he or she can solve your mystery, too. You probably won't bring all this information on stage immediately, and some of it may never show up, but you need to know it. It's how you give your character depth.
If you're writing a series, this bio is even more important because some stuff may not matter until the third or fourth book, or even later. Publishers and agents love, love, love a series.
Lately, I've been moving from novels to short stories, and my thinking is changing, too. Maybe my attention span is waning, or maybe I'm just trying to go faster, but for short stories, it's all about the plot.
Remember, instead of 80K words or more, my short stories average about 4K, roughly 15 pages. Get in, get dirty, get out again. There's less room to present a complex and fleshed-out character. Unless you're trying to sell a story featuring a character from your series--which I've only done two or three times--you rely more on your premise, and that's more apt to guide your plot.
You need a character who will logically find herself in a particular situation. For a short story, once I have a premise, I start typing with generic names and see where those given circumstances lead me. I characterize the protagonist with action and his or her goal instead of with lots of description and back-story (both of which I tell my writing workshop students to leave out). If I go quickly and don't censor or force things, they will lead to the detail I need, and that often provides a plot twist, and maybe even a solution.
Let's say you're writing about a woman who qualifies as a "crazy cat lady." She has eight cats and has hidden her will somewhere in her enormous house. Cats suggest certain ideas: mice, purring, dogs, people who like or dislike them, people who are allergic to them. What if a supporting character loathes cats? What if she likes them but is allergic? Can you use that as a plot point, or even a clue? Maybe. It's a character detail, but it steers your plot. More and more, I discover details that flesh out the plot at the same time they delineate character, and when you get two for the price of one, it's even better.
As I re-wire my brain for short stories, I find that I'm writing them more quickly and maybe having even more fun. I'm fond of a few stories that have rich and complex characters, but several of them have never found a home except on my hard drive. The newer plot-premise stories seem to have more potential markets, so I can send them out with higher hopes.
That's a happy ending.