It is one thing for an aspiring young actress to make herself look older or less attractive for a role, but quite another for a 67-year-old to do it.
It is even more remarkable when that 67-year-old is Susan Sarandon, an Oscar- winning performer with an illustrious career and a reputation for her sizzling sexuality on screen.
Then again, she has often marched to her own drum in her career and personal life. And this is one reason the star of acclaimed films such as Thelma & Louise (1991) and Dead Man Walking (1995) said yes to Tammy - a road-trip comedy that entailed playing a deliciously delinquent grandma.
In a refreshing departure from Hollywood's usual tendency to have older characters look implausibly young, Sarandon gets the full ageing treatment for the part, including wrinkles, frizzy grey hair, prosthetic "cankles" to simulate swollen ankles and peppercorns in her shoes to make her limp.
"It's very liberating to look that bad,'' she says brightly at a publicity event for the film. "We just accentuated everything you normally hide. So it didn't matter if I was sweaty or if the lighting was bad - that was kind of cool. Well... we'll see if it works but at the time, I was, 'Let's just go for it'."
She adds matter-of-factly: "Actually, the character isn't much older than I am - she just doesn't have my make-up and hair people. And so she looks different."
Seated next to other members of the cast - including comedy veterans Melissa McCarthy, Ben Falcone and Mark Duplass - Sarandon is the life and soul of the press conference, which she presides over while cradling her tiny pet dog, a Pomeranian- Maltese mix named Penny, in her lap.
When a reporter begins a question by telling her she would "have to be one of the sexiest grannies to appear on screen", the room erupts in laughter after she interrupts him to query: "Who else was in the running?"
She then thanks him before admitting that it is ever so slightly ridiculous that she was cast as the mother of Allison Janney's character, who at 54 is just 13 years younger than Sarandon herself, and that Janney is, in turn, just 11 years older than her on-screen daughter, played by McCarthy.
"Actually it makes sense if you figure I had a baby at 16 and Janney's character had a baby at 16. It's totally possible.''
But she adds: "I'm bad at math. I've always had a problem. And so people's ages just don't resonate with me. If you'll look at my life, you'll see...''
She pauses here for comic effect and gets a few giggles as people realise she is probably referring to some of the much younger men she has been romantically linked with, notably ex-boyfriend and actor Tim Robbins, who is 12 years her junior.
"I'm like a free agent. I just travel in and out all over," says the actress, who has two sons, Miles, 22, and Jack, 25, with Robbins, and a 29-year-old daughter, Eva, from a previous relationship with Italian film-maker Franco Amurri.
"I've not done anything on the right schedule or been with the people, the right… whatever,'' says Sarandon, who is set to become a grandmother for the first time when her daughter gives birth this month.
This is why she did not think the improbable age gaps in the film were such a big deal. "Really, there are other things in the movie that are equally questionable," she jokes. "You either make the leap or you don't."
After completing the movie, she was reminded of another road-trip film she made - Thelma & Louise (1991), which earned her the second of five Oscar nominations before she took the statuette home for Dead Man Walking (1995).
Sarandon recently tweeted a picture showing her and co-star Geena Davis recreating a well-known scene from the film in which their characters snap a photo of themselves.
She cheekily captioned the Tweet - "Inventors of the #selfie at it again. #ThelmaAndLouise" - and it went viral, prompting some to wonder if she and Davis, who got an Oscar nomination too for that movie, will work together again.
Sarandon says: "I didn't think of that but that's a great idea. I'd love to work with her again."
She pours cold water on the idea of a Thelma & Louise sequel though, given how the first one famously ended.
"They had knocked around some ideas for a sequel to Thelma & Louise but they were so ridiculous. I remember at one point saying, 'Well, what would we do?' And somebody said, 'You'd collect a big cheque, that's what you'd do'.
"But I don't really know how you'd resurrect that," she says. "I love Geena but at the moment, I don't know what that sequel would be'."
Tammy is showing in cinemas here.