"It was a bit like doing King Lear. There's so much dialogue, mainly me, blistering on and on and on," the 42-year old muses.
Beckinsale had previously starred in a 1996 BBC production of Austen's Emma, which was overshadowed by the more Hollywood version starring Gwyneth Paltrow.
She likes that this is the first screen version of this Austen's story.
"Austen didn't publish Lady Susan because she thought it too wicked for that period. Usually amoral women ended up burnt in a fire or dying of syphilis. Yet Lady Susan gets away with it, and given the constraints on women during that period it makes you cheer her on. Austen characters aren't instantly likeable or instantly good people. You have to really understand their perspective and why they feel justified."
Beckinsale welcomed a reunion with Stillman who had paired her with Chloe Sevigny in 1998's Last Days of Disco. The trio have remained friends and Stillman says casting Beckinsale and Sevigny (as Susan's best friend, the American Alicia Johnson) allowed the film to be financed.
"Last Days of Disco was a social comedy about a tiny strata of people I'd never come across. I'd never done an American accent, so I had to stalk Chloe and listen to her on the phone. With Love & Friendship I was back in my comfort zone since I grew up in England. It was Chloe who was out of her depth and it was nice to be much less mean to her this time!"
In fact the pair are fellow schemers as Lady Susan squarely aims her intent at her sister-in-law's brother, the dashing, available and very rich young nobleman, Sir Reginald DeCourcy, played by Australian actor and Twilight star Xavier Samuel.
Samuel admits he is a Beckinsale fan and can imagine why his character would fall for her charms.
"Sir Reginald thinks he knows a little more than he does," says Samuel. "He has a kind of cynical perspective about Lady Susan's arrival and is curious to see if her reputation is as grand as is it seems to be. Then he winds up falling madly in love with her, despite his previous perspective."
There's no doubting Beckinsale looks delectable even if she says it was difficult getting through doorways "wearing a hat the size of a cartwheel. I was surprised that we had a lot of our costumes made from scratch. I turned up for what I thought was a tiny project to find extraordinary costumes that really paid attention to the characters.
"I think Chloe had the most outrageous ones; there were more boobs on Chloe's dresses! They were lively and very concerned with colours. Whit didn't want to go with the usual Jane Austen novels; he wanted to go for the saucier French fashion of the period."
The thin-as-a-whippet actress also has the fifth Underworld instalment, Blood Wars, in the can. She says the two roles at least have one similarity: skin-tight apparel. "The latex in Underworld is also like a corset. I'm rarely out of a corset, it's just my lot in life," she sighs.
"In Underworld I also wear embarrassingly tight trousers. But I feel lucky in my career to have been able to play really strong women."
Who: Kate Beckinsale
What: Love and Friendship
When: At cinemas from Thursday
