So what does it mean, doing historical cosplay?
Let’s use an example near and dear to most of the seamstresses here: Beauty and the Beast.*
If we were to do straight cosplay, we’d take our favorite dress from the movie and play with making it as screen-accurate as our talents/resources can make it. We’d analyze as best as we could the construction and movement to figure out pattern parallels and fabrics.
Straight cosplay of this, I’d get two/three basic fabrics: a mid-weight cotton fabric as close to the blue as I can color-match and a lightweight white cotton to use for the blouse and skirt. I’d either reuse the cotton with some lace for the petticoat, or get another white fabric closer to the medium weight of the blue fabric, and again, a light lace edging. I’d use a shawl-collar blouse pattern and adjust the sleeves to be ¾ length bishop sleeves, and probably adapt a dirndl style dress pattern for the dress. The skirt looks like a gored A-line, easy enough to use or draft, and the apron is a trapezoid attached to a strap. I’d make the petticoat from the skirt pattern, hemming it an inch or so shy of the mid-calf length of the main dress. I’d grab black ballet flats for her shoes, and starch or interface a tube of the blue fabric to make the hair-bow. And to get in or out, since there are no fasteners visible in the gown, I’d probably set an invisible zipper into a side seam, where it’s least likely to be noticed.
That’s a straight cos-play. At the end, if my skills are decent, I’ll end up with something that looks pretty close to the screen version. So what happens when you want to do a historical cosplay?
To start with, a historical cos-play is about finding what fits the time the movie costume should be set in, your “period”. What period is Beauty and the Beast set in? The movie itself is a Disney anachronism stew. There are references to the Eifel Tower (1880s), and yet the Beast is a Prince (French Revolution in the 1780s-1790s kind of stopped that possibility by killing off everything royal and most of the nobility to boot). Cogsworth mentions Rocco architecture (1700-1770s) in his palace tour, and Gaston shoots game with an approximation of a blunderbus. I haven’t spotted an official Disney time designation in my admittedly limited research on the subject. And since Disney’s official dating system is often times relatively arbitrary, that’s not the most useful approximation either. About the only thing I’m going to rule out is anything after 1789, because of above-mentioned revolution doing a number on anyone calling themselves royalty in France. I’m also going to rule out anything much before the Enlightenment (1715 start), based on general technology level at a rural village. Please note, I am not a historian, or even history-adjacent, and my preferred periods of garb don’t skew to this era normally. My winnowing here is less than perfect.
What it does let me do is jump to the next piece of information: the costumes themselves as drawn/made. What is the silhouette, what construction details can we make out, and do they look at all like anything from the general period we slotted in? I started looking at portraits and fashion descriptions from 1700-1780 to see if I could find a decent era. I’ll admit, I wanted to skew it earlier in the period for the part of me that doesn’t want the epilogue to the movie to be, “And three years later the castle was stormed yet again by the villagers, and Prince Adam, Princess Belle, and their child were put to death as part of the Reign of Terror to eliminate this cadet line of the house of Bourbon.” I also am not super fond of the wide panniers look of court dress in the latter half of the 1700s.
Belle’s dresses are the blue and white peasant dress worn at the beginning of the movie, a green dress and a pink dress with very similar lines that she wears day to day at the palace, and of course, the iconic gold gown of the ball scene. Portraits and art being what they are, we don’t have many images of the working class from this era. Looking at a few paintings, there are some ways to tweak her dress into a generalized late 1600s-early 1700s peasant dress: lengthen the skirts, of course, and change the sleeveless top over a blouse into a bodice with a white shift/blouse underneath. Look at this picture, for example:
Not perfect, but there’s at least a similar silhouette and colors. This is a painting from 1738. Note that there’s still relatively full sleeve and they end at the ¾ length. There’s a fichu that echoes Belle’s shawl-collared white blouse.
The skirts are fuller, as is the apron, and the hair and makeup definitely don’t match Disney’s 90s girl ponytail that Belle sports throughout the movie. But if you tweak this dress by doing it in Belle’s blue, and change the bib apron here to Belle’s half apron, you’d be close enough that most people would make the connection. You could also make an argument that a governess was a transitional level in class rankings, and that a governess’s dress would be higher status than Belle, and might have more fabric to play with. Fabric is expensive, very full skirts means more money is spent. I’d still make a historical cosplay of Belle with fuller skirts than the movie demonstrates, but perhaps not as full as this dress demonstrates.
Switching to the castle gowns, how do they stack up against 1720s-1730s dresses? Remembering here that we can make some lee-way on exact dating because if everyone in the castle has been enchanted for the past 10 years, they don’t have the latest fashion dictates from court yet. Belle’s gowns would be based on what they had and how much they and she could (or wanted to) modify them. 
Portrait dates to 1729
Hey, not too bad! These are round gowns (closed in front with no petticoat visible). Fitted bodice, and sleeves are closer to the body. Compare the two pink gowns especially. It’s hard to see with the book in her hands, but other screen-shots of her bodice show a thin pink stripe straight down the mid bodice that mimics the portrait’s front lacing. Belle’s skirts are less full here, but they are as well when you combine the blue dresses. I can’t find many narrow skirts like this until the 1800s, with the Regency/Empire gowns, and that’s 1. After the Revolution which I’m not going, and 2. Screws up the waistlines even worse. We can handwave the skirt fullness a bit. As for the shoulder puffs on the green dress and the full-length sleeves? Not a clue, those seriously look more like late Victorian than Enlightenment anything. I’d say you could easily base a round gown off of these dresses and mirror some of Belle’s attire. Would it be as easily marked out as cosplay and Belle based? Probably not, but these dresses also aren’t as iconic Belle as the blue dress or the gold gown. 
And what about the Belle gown to end all gowns: the yellow/gold ballroom gown? It certainly shares the very fitted waist of the other dresses, and adds in a much fuller skirt. How does it stand up to something from the 1730s? What about the sleeves/neckline? How do they compare?
1723 Portrait of Louis XV in front of a portrait of the Infanta Mariana Victoria.
Look at the neckline hovering off the tips of the shoulders! Look at the rich golden tones! The short self-sleeves over the upper arms, even if they’re followed by lace to the elbow. There’s the basque point to the tight bodice, the lack of any visible chemise, and the more rounded fullness of the skirt, not yet at the wide panniers later in the century. This portrait shows an early example of a robe de cour, which became the standard of dress in the French royal court during the 1700s. Earliest robes had a wide range of skirt silhouettes, including the rounded cone/dome shape Belle’s dress demonstrates. And here I’m going to point you to someone who has done a ton more research on the topic than me and talks about what a robe de cour is at great length with a ton more pictures: http://thedreamstress.com/2013/09/terminology-what-is-a-robe-de-cour-or-grand-habit/
How would we turn Belles’s golden gown into a 1720s/30s robe de cour? The main thing I would change would be the sleeves: modify the drape over her shoulders into short cap sleeveshanging off the edge of her sholders, in the main dress fabric, and then add in lace lower sleeves to the elbow. Get rid of the gloves, and attach a train from the waist. I’d keep the more conical/dome skirt shape because it fits the earlier time period’s fashions and stays visually more true to the movie’s design. I think it would still feel very Belle, and still read as the movie costume, but with a bit closer hewing to historical gowns.
*As in, I’m pretty sure there was a mad Skype session where we divided up the costumes and who got to make which one. I snagged the green library dress.
The Sewing Room of Requirement 