How to make minor characters that move the plot forward.
First, you need to know your plot. For some people (like me), this means you have to finish the first draft of the book and figure out what the story is actually about. Other people can plan ahead and do this before they write their minor characters. All I can say about that is thbbbbbt.
So you know your plot. Next, make sure you know the reason why the event with the minor character needs to happen.
Minor characters are a detractor–a distractor–from the story. Readers don’t care about minor characters as much as they do about the major characters; if they do, you’ve done something wrong. Every time you bring in a minor character, you’re pulling the reader away from the main characters, so the reason you bring in the minor characters had better be damned good.
Okay, back to the example: “A man who knocks his coffee into the heroine’s lap, causing her to bump into the man who becomes the romantic interest.”
Let’s say the main plot is about a heroine (Marla?) who lives life passively, dreading both the good and bad things that happen to her, because all she wants is peace and quiet after her horrible childhood. She meets a cute guy (Hank?) who lives his life to the fullest–food, sex, alcohol, rebellion, travel–and can’t get him off his mind.
How should the characters meet?
The cute guy, Hank, would never hit on our heroine, Marla. She’s uninteresting. Marla would never talk to Hank–he’s trouble, the last thing she wants.
So they meet by chance. A man in a diner knocks coffee into Marla’s lap and she backs into the Hank. But what about the man with the coffee (Don)?
He could be a nobody. He could be a nobody with one interesting detail. Or he could mean something. To make a minor character mean something, you have to get at the “so what” of the story.* Here, I’m going to say the “so what” is “Chance giveth; chance taketh away. Determination is what makes life have meaning.”
Who should Don be? We could make him a gentleman, and give Marla a choice between following up on the accidental meeting with Don or the accidental meeting with Hank. Don, instead of disappearing from Marla’s life, could call her later on and take her on a date that leaves her flattered but cold. –Chance led both men into Marla’s life; her determination drew her to one over the other.
We could make Don a sweet, stuttering geek. We could make him an ex-boyfriend from 7th grade. We could make him the cop who pulled over Hank last week for speeding in a heavy fog. We could make him a trucker who doesn’t even notice what he’s done–while Hank gets pissed off for being bumped (at least noticing Marla).
The point being that minor characters, no matter how minor, are related to the major characters (and to the plot) in some way, either through an actual relationship or through an attitude they have toward the major characters.
Something I like to do–especially with mysteries–is draw a “web” of characters. The main character is at the heart of the web. The major characters are arranged around him; the minor characters branch off whoever they come in contact with. Each strand of the web is a relationship (“Mother” “Head of Secret Cult MC is fighting”) or an attitude (“Hates MC” “Loves MC’s mother”). Extra connections tend to suggest themselves, even to the point of minor characters becoming major players or recurring minor characters later on (“Head of secret cult loves MC’s mother” “Mother hates MC”).
Your minor characters should be fun. They should introduce surprises–even to the writer–and threaten to change the plot, right down to its bones.
Otherwise, you can just have Hank spill his own damned coffee in Marla’s lap.
*I think getting at the “so what” is the heart of my writing problems, so this comes up a lot with me. “So what” isn’t theme, by the way. It’s more akin to “the moral of the story.” For example–in “Little Red Riding Hood” the “so what” goes something like “The cost of loose behavior is more than you expect” or “Don’t talk to strangers.” One of the themes could be “sex” or “death” or “women need to be rescued.”