It's my belief that this happens when the author has not flushed out her supporting characters fully enough. An easy thing to do as we spend so much time on our MC, plot, world, and a million other things that go in to creating a seamless piece of art. Say we have a character that is (we feel) necessary, but not "on stage" very much. (Sorry my theater obsession is showing again).
Anyway, say we have this character, and she's - oh I don't know - a mermaid. Her purpose in your manuscript is to be a beautiful temptation to the merman the MC is after. So she's pretty, but not much in the brains department. Just a device to make your MC realize she's jealous and must have actual feelings for her fishguy. So do you need to spend time really thinking about Mermaid Girl? Is this a waste of time and energy? Bzzz! You better believe you need to do that. Otherwise, guess what? The reader will see it as what it is. A device. Your mermaid is no better than the brainless blond bimbo stereotype with a tail.
Here are a couple of tips you can use to get in that mermaid's head:
1. Write a page or two from her perspective.
See what makes her tick. Why is she coming on to this guy? What does she think of the girl he's hanging around with? Does she have any other guys she likes? How does she feel about being so attractive? Does she even know it? You get the idea.
2. Give her an anti-stereotype.
Give her a flaw or just a characteristic that isn't something that's immediately apparent. How about a fear of water? Say she's the blond-bimbo instead. Maybe she's the biggest brain at the school. A cheerleader? How about a cheerleader who's a klutz?
3. Put her in your MC's shoes.
Say she's the MC. How would she handle the main problem differently? How would it change the book? Probably it wouldn't work the same way as it's our character's choices that drive the story. But this exercise will not only get you in the Mermaid's head, it will also strengthen your understanding of your MC.
Okay, now you're a little more prepared. Re-write the scene. What happens? Does it change anything? It might. She might have an actual conversation with your MC that makes her think twice about this mermaid vixen. She might surprise you and get up and slap the merman. Or maybe she actually has the hots for the MC. See, if you had looked at her as a device instead of a full fledged character you might have missed some great potential plot points. Or at least a nice unexpected moment. And overall it will bring more dimension to your writing. Trust me.