It involved a scene with her co-star Liam Neeson, who plays her bullying outlaw husband Clinch Leatherwood. Theron was required to whack a trouser-less Neeson over the head and cheekily place a flower where the sun doesn't shine.
"I said what any healthy woman would say," recalls Theron, giggling. "'I get to see Liam Neeson's butt!' That's what happened. I can think of so many worse jobs!"
Born in South Africa, Theron has been steadily working for the last two decades, famously arriving in the States aged 19 with $400 to her name and a boat-load of ambition to make it in films.
Yet, despite the plaudits for her performances in heavy-hitting dramas, particularly as serial killer Aileen Wuornos in Monster, for which she won a Best Actress Oscar, Theron's isn't the first name that springs to mind when it comes to comedy.
Fully aware of this fact, she says she "begged" Ted creator Seth MacFarlane, who helms and also stars in A Million Ways To Die In The West, to give her the role of mysterious gunslinger Anne.
"I've been interested in comedy for a while, but it's been tricky because audiences know me so well doing something different," admits the 38-year-old.
"In fact, what also interests me is odd comedy. Those are rare. The combination of this script and Seth directing was a slam dunk for me. I started begging instantly."
Odd is a good description of this film.
MacFarlane plays Albert, a sheep farmer who feels out of sorts with the hard times he lives in. When his girlfriend Louise, played by Amanda Seyfried, dumps him for the smug Foy (Neil Patrick Harris), he becomes friends with Anna - who advises him on how to win fickle Louise back, not realising Anna's connection with her husband Clinch. There's also a song about moustaches, urinating farm animals and plenty of toilet-humour gags.
While the jokes may not be everyone's cup of tea, it's clear that stepping away from serious drama was freeing for Theron.
"I loved watching this movie, because it felt like I was seeing myself,"says the actress, who has a two-year-old adopted son, Jackson, and is dating Sean Penn.
Going into the unknown could be daunting prospect for some, but Theron was quite clear in how she should approach the comedy script.
"The worst thing you could do is show up and think you're going to be funny, because the universe doesn't work that way," says the star.
"If the writing is good and you have good director like Seth, who is at the top of his game, the comedy will naturally come from that."
Likewise for Theron, who shaved her head for the upcoming Mad Max film and piled on the pounds for 2003's Monster, the chance to play a version of herself, instead of someone alien, was a nice change.
"There's an idea that actors have that unless you're completely transforming and disappearing into a character, you're not really doing your job," says the former model.
"There was something refreshing after 20 years of doing this; watching a movie, seeing and hearing myself and feeling like that was enough to tell that narrative - that there's nothing wrong with that."
Theron, the first South African to win an Oscar, is appreciative of how things have turned out for her.
Her early years weren't always easy as her father was an alcoholic. Aged 15, she witnessed her mother shoot him dead after he tried to attack her in a drunken rage.
"I'm completely aware of how blessed my career and my life has been," she says.
"My 22-year-old self would probably just be jumping up and down going, 'Woohoo, we're still doing it! We're not a waitress! We kept going!', so I'm grateful for that.
"You have to be in this business."
l A Million Ways To Die In The West is in cinemas now