… do it on purpose.
Because I’m more familiar with princess movies than, uh, a lot of things (what can I say: I grew up on Disney) we’re going to do the comparison using princess movies. But I’m sure you can find other examples in sci fi, or action, or any number of other genres.
Three princess movies that all contain fairytale magic, but are set in a sort of medieval/baroque/otherwise “period” era: Ella Enchanted, Cinderella 2015, and Beauty and the Beast 2017.
You know, right off the bat, that not everything is going to be realistic, because duh, there’s magic involved. At least one of the main/supporting characters is going to be inhuman. But the attitude that each of these movies takes toward that old-timey setting is very different.
You’ve got Cinderella 2015, what I’d call the middle of the spectrum. Most of the sets, the costumes, are based solidly in an 1800s French sort of style. The only obvious anachronisms are in the stepmother and stepsisters’ costumes, which are clearly done on purpose in order to show how different they are from everyone else in the story. And the fairy godmother, while she has little sparkly wings attached to her dress, is more glamorous than everyone except Cinderella — with LEDs in the skirts! — but in a way that doesn’t stick out badly, even if she only has one scene in the movie. The dialogue and setting and costumes mesh pretty well to provide that historical-feeling ambiance. (You can read a more detailed analysis of the costuming in Cinderella 2015 here.)
Then there’s Ella Enchanted, which has the fairy godmother in a mini skirt and go-go boots, and Eric Idle narrating, and Hattie as the president of the Prince Char Fanclub (zomg u guyz!), and the main character singing a Queen song during karaoke. None of this existed in the book this movie was loosely based off of, but the movie doesn’t care; it’s delightedly zooming its way through a story that can be whatever it wants, because there’s magic and elves and ogres, darn it, it doesn’t have to be realistic. The dialogue, setting, and costumes are all consistent in this regard. So the anachronisms, instead of being annoying, are entertaining. (See also: A Knight’s Tale, even though that one doesn’t have any magic involved.)
And then you have Beauty and the Beast 2017, which is very clearly trying to be modern in its sensibilities but historical in its setting and costumes, Which … really doesn’t succeed, because the modern sensibilities bleed over into the costumes, and not in a way that feels like it was done on purpose. Belle wears period clothes throughout the film except for the ballgown in the iconic scene. The contrast is pretty jarring, especially when you realize that that yellow dress wouldn’t look out of place in a high school prom; and that kind of dissonance is usually reserved for the antagonists, not the main character (c.f. Cinderella). Then you also have the dialogue, which in some moments is lifted straight from the original movie, and in some moments feels like it could have been lifted straight from the original movie, and in some moments has words straight from 2017 that just immediately ruin the moment. (At least for me: the Beast saying the phrase “too touristy” was a definite nope.)
I won’t even begin to go into the dance choreography.
The thing is, there’s always going to be something that doesn’t quite mesh with everything else. And that’s okay. But it’s like writing an essay for English class. If you want there to be a Solid Theme (i.e. Belle being “not like other girls”), then everything you do has to be related back to that theme. Make her other clothes more modern too, instead of just the ballgown. Or, if you want the solid theme to be “this could have taken place in a palace not far from Paris in the 1700s”, then even if you’re stuck on Emma Watson not having to wear a corset, you could at least make some kind of nod to the fashions of the time instead of that .. ruffled, cake-layered … thing. But the key is consistency.
I don’t care — and your audience won’t care either — whether you go full-on Research Mode and toss in as much trivia/jargon/whatever from that time period as you want, or whether everything is neon lights and karaoke. Just as long as it’s entertaining, and as long as it’s consistent.