Her role, baring practically all in the Victorian period lesbian drama, provoked predictable outrage in some circles.
"I had a vague inclination that where there was a dildo involved there would be a bit of a hoo-ha!" she says, with a wry smile.
"But I didn't expect all the papers to lead on the dildo, or my butt cheeks, or whatever. But no, the reaction hasn't put me off showing my boobs!"
The 26-year-old daughter of Avengers star Dame Diana Rigg and Scottish landowner Archie Stirling recently starred in the new series of Poirot.
After all the exposure in Tipping The Velvet, this time she showed barely an ankle playing a woman convicted of killing her husband.
But the true character of Rachael is a little more down to earth.
"To keep normal and to keep fit. I play football," she says. "I play with some good girlfriends. Our team is called Frisky Town.
"We train every week and we play against other teams of lawyers, journalists, anyone really but, God, some of them are ugly bitches! We are so much prettier than they are!"
Rachael's upbringing was far removed from the muddy football pitches frequented by Frisky Town.
As a child, she spent hours riding horses on her dad's Perthshire estate. Archie, 61, was a dashing, former Scots Guard officer from an aristocratic family.
He had two sons, Willie, 38, and Ludovic, 36, by first wife Princess Alice, the Duchess of Gloucester's niece. The marriage ended in divorce in 1977, the year that Rachael was born.
Stirling married Dame Diana five years later and Rachael refers to Willie and Ludo as her "elder brothers" - fondly recalling the idyllic times they had together in Scotland when she was growing up.
But her father had a very public affair with the much younger actress, Joely Richardson, resulting in the breakdown of his marriage with Rigg and influencing Rachael's decision to go to boarding school in Buckinghamshire from the age of 11.
She admits she suffered "some bullying" and describes some of the older girls' behaviour as, "proper Cruella DeVille style stuff".
But she was happy to miss the disintegration of her parents' relationship. They divorced in 1993.
When asked if she wanted her daughter to follow her into acting, Dame Diana always said "No, no". But Rachael had other ideas.
After school she went back to Scotland to study Russian and Art History at Edinburgh University. Before finishing her degree, she had already featured in two movies.
The first was Still Crazy, with Bill Nighy, Timothy Spall, Juliet Aubrey and Jimmy Nail.
Rachael says: "The director asked me if I would quit university. I said no and so they flew me down to Pinewood for filming and back for lectures.
I played Juliet's daughter and fell in love with a young guy in a rock band. My performance was sh*t. It took me 10 takes to walk through a door, but I learned a lot."
She went on to star in Complicity with Jonny Lee Miller then, after graduating, appeared in Another Life, about Edith Thompson. Her character in the latter was hanged, and bizarrely she again faced the gallows in Poirot.
For her latest role, Rachael says preparation was the key.
She says: "Filming the execution was the most difficult thing for me because we did it as accurately as possible according to the books and notes of the time.
"It took 45 seconds in those days for the woman to be walked from her cell to the moment when she was hanged.
"It was so realistic that no matter how many times I told myself it was just a film set, when your legs are strapped together and that trap door under you opens, it is very hard to believe it isn't real.
"The hanging scene stayed with me for a long time. That was the last day's shooting and I was on my own in the studio, pretty lonely and miserable."
You can't imagine Rachael being lonely but having recently ended a two-year romance, she insists: "I'm not going to have another boyfriend for a very, very long time. The next few years are about getting as much experience as I can and working as hard as I can."
There is no doubting her passion for acting, but she admits when she began her career she was so horrified by the idea of anyone finding out who her mother was that she kept her parentage a secret.
She concedes: "If I felt that my name was opening any doors for me then I would go completely in the other direction.
"Nobody even knew who my mother was until halfway through filming Still Crazy, when a newspaper ran a piece about me being 'undercover' on the film.
"I was very touchy about it because I felt it was very unfair to make comparisons between myself and an actress of my mother's experience."
But Rachael admits she has grown up a lot in the last few years.
She says: "Tipping The Velvet helped me and now I feel I do know what I am doing and I have more experience. So now if people say I look like my mother, or sound like her, then I am proud."