In a sense, I’ve already answered that question, since my current side project, apart from the fantasy, is a venture into Regency romance. I’ve always been a big fan of Jane Austen and Georgette Heyer – very different styles, but both endlessly rereadable – and my very first attempt at novel writing, many moons ago, was a full-on Regency. That effort was banged out on an old manual typewriter, and I got maybe three-quarters of the way through before life overtook me. It now lurks, unloved, in a bottom drawer, and I haven’t dared to read it again. I’m quite sure it must be execrable.
Fantasy and Regency might seem to be very different creatures. One is a made-up world, with the only limitation being the author’s imagination, focusing on battles and monsters and world-threatening peril, not to mention magic, of course. A Regency focuses on a much narrower field of action, which may be just a few towns or villages in England, with one not-very-earth-shattering objective — to marry off hero and heroine. There may be adventures and high jinks, but generally a Regency is light-hearted fluff.
But in both cases, the characters are tip-toeing through the same minefield — the rules of their world. In a fantasy, the rules are made up by the author (you can use magic, but only if you’re carrying a certain gizmo, or use the right words). In a Regency romance, the rules are those in effect in the real world at the time — the social rules that constrain well-to-to families, with dire consequences if breached. So young women must be accomplished and knowledgeable, but also demure. They are brought up to run large households, yet must always defer to their father, brother or husband. They may speak several languages, but must never express a political opinion. Woe betide the young lady who dares to waltz in public before being approved by the patronesses of Almacks.
For those who still think that fantasy and nineteenth century manners have nothing at all in common, I refer you to a couple of examples of books which gloriously mash together the two genres. Tooth and Claw by Jo Walton is an only partially successful attempt to blend the Victorian social culture with dragons. Temeraire by Naomi Novik thrusts dragons into the middle of the Napoleonic wars, and although there are certain logistical issues (nations can call on fighting dragons, but somehow history has turned out pretty much the same? Really?), the first few in the series are quite glorious, entirely dominated by the rather bookish dragon, Temeraire himself.
Footnote: Authors Answer is the brainchild of blogger Jay Dee Archer, of I Read Encyclopedias For Fun. You can read the answers to this question by his eclectic bunch of authors here. More recently, Erica Dakin, of the Theft And Sorcery blog, has been answering the questions independently. You can read her answer to this question here.