Vanessa Kirby shot to global fame as young Princess Margaret in “The Crown,” winning plaudits for her sultry and tragic portrayal of the British queen’s headstrong younger sister.

In “The World to Come,” out Friday, the English actress plays another complex and passionate woman grappling with the impossible pressures and conventions of her era — this time, the 19th-century US frontier.

The movie follows two farmers’ wives trapped by a brutal winter in pre-Civil War America, with only their neglectful husbands for company, until an unexpected and forbidden affair develops between them.

“I love women that are sort of ahead of their time, in their thoughts,” Kirby told AFP.

“I didn’t want her to seem like a wide-eyed ingenue trying to work out the world… I wanted people to imagine that she has a modern energy.”

Just as Princess Margaret was forced to split from divorced war hero Peter Townsend, history is full of big female personalities who were “caged in” by society’s expectations of how they should act, said Kirby.

Women like the fictional pioneer farmer’s wife Tallie “had to straitjacket themselves” and were afforded “absolutely zero choice of what you do with your life, or your time, or your love, or your heart.”

“It’s also a sort of ode to the women that may have tried to go beyond the system, and have sacrificed their life for it, ultimately,” added Kirby.

The likes of 2005’s “Brokeback Mountain” have depicted same-sex historical wilderness romance before.

But big-screen historical lesbian romances have “taken a really long time to catch up,” said Kirby.

A recent crop has included acclaimed French drama “Portrait of a Lady on Fire,” and Kate Winslet’s “Ammonite.”

“It’s a course correction — a lot of people are sort of saying ‘Ah, so many corseted lesbians all of a sudden!’” joked co-star Katherine Waterston.

“I think actually it’s just a few. And that’s a problem — there’s lots of stories that need to be told.”