Another long one for #160! And no, you don’t have to become an expert on everything your characters do, but especially if it’s an integral part of a protagonist’s story arc, at bare minimum you need to not make mistakes representing it. And you may want to say stuff like “OK, so what if my character is an exception?” For instance, what if one of your test readers says your police officer character wouldn’t do a certain thing because that’s not part of the protocol for the incident they’re responding to, and you want that character to break the usual rules and do it anyway? The answer there is you need nuance. You need someone to call it out or later acknowledge that the officer shouldn’t have done that or have the character’s inner monologue note that they’re ignoring protocol. You don’t want it to look like you just didn’t know how to write this.
Think about your own hobby or profession, and then think about the silly things you’ve seen in movies/TV or read in books about the way it’s represented. It’s pretty obvious, isn’t it, when these things are phoned in! But it’s not just about not wanting to get laughed at by experts. It’s about respecting your own work enough to represent an accurate picture of the character’s life. In researching these things, you may even find some insight that will help your story feel more authentic in general!