We're in Edinburgh where she is promoting her latest film The Private Lives Of Pippa Lee and on the first two occasions I assume she's putting it on for comic effect.
On the third, I can't help but point these moments of received pronunciation and she looks startled.
"I know," the 43-year-old says confessionally. "It's because when I'm around you people I feel just like I have to speak like this because I hate my accent. I feel very American around you guys."
She starts making a whining sound and adds: "That's what I sound like to myself when I'm around you."
Robin's father is English so she had a head start with the accent and the California-based actress admits that - after putting on her costume - adopting another accent helps her get into character.
"Clothing is so important to me when finding a character. It changes your walk, the way you hold your arms, the way you sit: it's maybe the most important thing. Of course, if you have an accent that's even better."
In her title role as Pippa Lee, a 50-year-old woman heading for a mental breakdown after moving to a retirement village with her older husband, Robin didn't have to feign an accent. But her clothes, she says, were instrumental in getting herself into the prim and proper role.
Given starched beige trousers and plain blouses, her on-screen wardrobe is eye-catching in its plainess.
"It was genius picking those very conservative, wardrobe pieces for me, and the curl in the hair aged me too.
Although close to her character in age, the almost-50 actress says the clothes really helped her get into the part.
"I really felt her, every day, because I wear this," she says, pointing at her skinny jeans and tailored leather jacket. "Always have."
In the film, Pippa has an affair with her friend's son, played by Keanu Reeves, who rescues her from her troubled past and the suburban drudgery she ends up living in.
Pitched to movie audiences as an age-gap relationship film, in real life Keanu is actually a year older than Robin. She admis that kissing him on set wasn't strange, because of the on-screen chemistry.
"He was so perfect for it. I feel like we were blessed to have him do it, because it didn't feel strange. The dynamic was already there. To me, that's chemical," she says, her voice inflecting upwards as if asking a question.
"When a director knows, when they walk in, that that person clicks, and we just had it," she continues.
"We needed Pippa to be seen and he's the first person that actually sees her for who she is. It's beautiful as it's like she's then set free, allowed to blossom."
Robin admits that now her children, daughter Dylan, 18, and son Hopper, 16, have grown up, she feels able to fulfil her potential in much the same way.
"The kids are grown - one's off to university - so I'm a free bird," she says with a joyful glint in her eyes.
The actress, who is married to Oscar-winning actor Sean Penn, is aware that having children put her out of the spotlight. Riding high from playing love interest Jenny in Forrest Gump, Robin was offered big roles in Robin Hood: Prince Of Thieves and The Firm but turned them down so she could have and raise her children.
"I've no regret about it," states Robin.
"When I had my kids I always knew that was the priority, and career always came second. I knew by making that choice to be a full time mom and turning down commercial movies took me out of the game, I knew that.
"I knew that would happen and I was absolutely fine with that. I did it very willingly and happily and loved it - loved raising my kids."
"But, you raise your kids - they're 16 and 18 now - so I'm like 'Go!'," she adds with a smile, doing a pushing motion with her hands.
With recent roles in films such as blockbuster State Of Play and her forthcoming lead role as Pippa Lee, Robin says she was relieved to come back to a successful career.
"There's a bit of a resurgence happening, a bit of a rebirthing," she says, admitting that she may be being offered these roles precisely because she's been out of the spotlight and safe from the typecasting which may have ensued.
Now she wants to continue taking on challenging roles and has already agreed to work again with The Private Lives Of Pippa Lee's writer and director Rebecca Miller, who she describes as "an original".
She would even like to do some travelling, and is prepared to rough it.
"I want to travel more," she says dreamily. "I always did travel before I had kids, around Europe - there are a lot of places I want to go. I want to do South America. I would rough it while I went down the Amazon - definitely."
Whether she'll include husband Sean in those plans is not known. The couple have had a rocky year, with him filing for divorce then retracting the petition immediately. He has since pulled out of two film projects in order to spend more time at home, reportedly to patch things up with Robin, but she refuses to be drawn on the matter.
"Thank God somebody's staying with the kids," is her only response - said in jest - to the news.
But one thing is for sure, Robin will not play the victim.
Talking about playing her character Pippa, who splits with her husband during the film, the actress says: "During rehearsals I remember saying, 'I don't want to go into the victim' because you've seen what's been inflicted on Pippa. I said I didn't want to play that victim that wallows in the, 'Oh woe is me and I never had what I wanted out of life'."
In fact, Robin seems stronger than ever and she says she's ready to take on the world.
"You know when you're young, you're not scared, you don't have that fear?" she asks.
"You haven't had those experiences that caused the fear of what may come? I think I'm going back to that non-fear place. It's weird, it just happened right before Pippa. I just thought, 'What are you wasting your time for? Just go for it, just do.'"
Extra time - Robin Wright Penn
Robin was born in 1966 in Texas but she grew up in California.
Her breakthrough film role was the female lead in cult favourite The Princess Bride, which also starred Fred Savage and Billy Crystal.
The younger version of Robin's character Pippa Lee is played by Gossip Girl star Blake Lively in the film. The two met only briefly but studied each other's camera tests to pick up mannerisms from each other so there was some continuity between them.