Head hopping is when a writer changes the viewpoint character in the middle of a scene, or paragraph.
“Why would you go to so much trouble for some guy you just met?” Anna frowned. She worried about her younger sister sometimes. It was like she thought the world was full of nothing but rainbows and unicorns. She just didn’t think.
“Why wouldn’t I?” Ellen straightened her shoulders, bracing for the storm she knew was coming. She hated when Anna butted into her business – which she always did – especially when she was making her feel bad for trying to do the right thing. “Being a cynic is not a virtue, you know.”
What are the rules?
The danger of head hopping is primarily that you run the risk of confusing your readers, and them losing interest because they don’t know what the hell is going on. You don’t want your reader to stop mid-paragraph going “Hey, wait a second, who said that?”. That is a sure way to break the magical flow that allows a reader to immerse themselves fully in a story, something any writer surely doesn’t want.
Most every writer or editor will advice against it. Some will go so far as to say that the practice should be banned, barred, blocked and burned in hell. Yes, it’s one of those issues.
Can it work? Because I really want to show both character’s emotions in my dialogue, you might say. Well, it has certainly been done. There are some very famous authors who do it regularly. But I would tread carefully. It takes a skilled writer to pull it off.
My advice is, use it if you must, but use it well, and be aware of what you’re doing. Use pronounced cues that lets the reader prepare mentally for the switch. Rules are, after all, meant to be broken, as long as you know what you’re doing.