How to make minor characters that show off the main character.
Back to the "so what." Out of the "so what" comes the main conflict--stories are about drama, which is based on conflict. --The main, story conflict isn't the same thing as the plot conflict. The story conflict is on a level of "Good vs. Evil" while the plot conflict is on a level of "Luke vs. Vader." However, the plot conflict relates to the story conflict; it's the concrete way the characters carry out the way the Big Ideas of the story smack into each other.
Going back to the original example: "The little girl with a kitten up a tree; she begs the heroine to save the cat. The heroine is afraid of heights. What will the heroine do???"
The "so what" is "Chance giveth; chance taketh away. Determination is what makes life have meaning." I'm going to say the story conflict coming out of that is "Should you take life passively or force it to be what you want?" and the main plot conflict is "Marla's fear of change vs. her desire for Hank."
At some point, we want to show that Marla isn't just afraid of the situation with Hank; she's afraid of everything. So we're going to make up an example that shows that off; we're going to put that kitten up the tree and see what Marla does about it.
Basically, the main character should have at least two possible courses of action at any given time. Will Marla climb the tree or will she walk away? Will she call the fire department? Will she find a competent-looking person along the street and ask them to do it instead?
The options should come out of different parts of the character, parts related to the main story conflict. (The main character should be one of the main moving parts that carries out the story conflict.) Marla is afraid, but she can also empathize with the little girl's fear of losing her kitten. Whose fear is more important? --Keep in mind the main character has to develop throughout the story; you can't make her do all her developing in a single scene.
Let's say we want to show how Marla tries to take matters in her own hands but gets burned--once bitten, twice shy. She overcomes her fear of heights but falls and breaks her wrist.
Fine, we've got all that figured out. But what about the little girl?
Again, we can make a random kid, a nobody. We can make a nobody with an interesting detail. Or we can make a kid who reminds Marla of herself at age six, and the time Marla lost a kitten up a tree and she never saw it again. Or an annoying kid, a bossy one, who makes Marla grit her teeth and ask herself why she's bothering to help the snot. Or a liar--there never was a kitten--making Marla feel even more betrayed by fate when she breaks her wrist. Or a kid who becomes more afraid for Marla than she was for the kitten in the first place: "Look lady, I think he's coming down by himself. Don't go any higher!" "No, I can do it!" Crash!
Again, the minor characters are related to the major ones, either by relationship or attitude. Minor characters can have their own internal conflicts that relate to the ones inside the main character, too--they feel the same; they're completely opposite; they can't believe anybody would care about the things the main character does; they feel horrified at putting the main character in this situation; they resent the main character for not seeing things from their perspectives.
The main thing is to remember minor characters have to touch the main character right down to the quick in order to elicit character development. Dealing with them has to make the main character hurt.