They fit when it’s the word the character would use. They fit when sprinkled. They fit when dumped.
They don’t fit because the author just wants to use them for the sake of using them. My mind goes to writing erotic versus porn…
…One needs a story. Some connection to the characters, some reason for them to be where they are, doing what they’re doing. Porn – open the door and go for it…
Just banging out those words without care.
Any word your character uses shows the reader something about that character. A character who says fiddlesticks when they dropped the roast dinner, well, they’re probably not going to say the f-word. Whereas someone who uses the f-word may not normally say poop instead of sh*t.
Although, that can change.
What was the context of the ‘f’ usage? Dropping the roast, burning their hand as they dropped the roast, then that person could very well say ‘poop’ in a normal situation.
For those ‘bad’ words you must also remember your genre, audience age, and your own comfort level. There’s plenty of ways to express everything and anything. Use what makes sense to you and your story.
It’s the figuring out and learning that’s confusing.