Well, hello, Dolly! "Who's that sweet thing in the pretty dress? Look at you! Thanks for comin' all this way!" It is such a pleasure to meet you! "Well aren't you sweet? Let's have a seat and rest our bones."

This is exactly the conversation Dolly Parton and I have as she walks into her rehearsal room in Nashville on a sweltering summer's day. But I could have written that exchange without ever meeting her, and so could you. Parton in the flesh is so exactly how one imagines her to be that as she sits opposite me, bandying about such Dolly-esque phrases as "You just need some good ol' horse sense!", "God gives us all a special talent" and, of course, "I'm just a simple country girl", it's hard not to wonder whether this is a Dolly impersonator. How can someone adhere so closely in person to their public image?

"This is just who I am!" she says, those wide eyes getting even wider, hanks of blond hair bobbing atop her head like leaves on a palm tree. "It's like what I always say: I may look fake but I'm real where it counts. Ha ha ha!" And with that she clutches a bejewelled hand to what is famously the most unreal part of her anatomy.

Now, you say that, Dolly, but it looks like a lot of effort goes into being who you are. How long does it take you to get ready in the morning?

"Not long, because I'm so programmed for it. First thing I do in the morning, after I have my breakfast and do my spiritual work, is put on my makeup and fix my hair, and I can do my makeup in 15 minutes," she says with a snap of pride. And that is pretty impressive considering it would take most women 15 minutes just to put on the amount of eyeliner she is currently sporting.

So do you look like this when it's just you and your husband Carl at home? "Well, of course! I don't want to look good for everyone else and like a slouch for him! So even if we're just driving around in our RV camper and hitting all the local fast-food joints I'll tease my hair and put it up in a little scrunchie."

I don't believe you eat fast food. Your waist is the size of my wrist. "I do have to watch it because I'm only about 5ft and you can't hold on to too much weight when you're so short," she says, suddenly terribly solemn. "So I stay on a low-carb diet pretty much through the week and eat what I want on weekends. And of course, nothing matters on Thanksgiving and Christmas! Ha ha ha!"

I hear you wear heels in the shower. "Oh that's horseshit! Ha ha ha! But I do wear high heels in the house – otherwise I can't reach the cabinets! I don't wear my heels in the shower or when I sleep – and I wear my little tennis shoes when I do my treadmill – but when I go out, of course I will. That's my look and that's how I'm always gonna be."

Today, Parton's look includes a stretchy purple cardigan, a swishy above-the-knee black skirt, cropped leggings and Perspex heels. It's an outfit that is strangely reminiscent of the ones worn by cool teenagers in 80s films. Parton, by the way, is 65. "I'm not trying to be fashionable. Never was! I got some clothes that I've had for 25 years. People say, oh, that's Dolly, Dolly does what Dolly does – and Dolly does! Ha ha ha!"

Or, as she puts it to a honky tonk beat in Country is as Country Does on her new album, Better Day: "'Cause I'm quite content with who I am/And if you ain't, well, kiss my ham."

Really, who could fail to love Dolly Parton? Well, aside from the Ku Klux Klan who, as if to confirm that it had a combined IQ in the single digits, has held demonstrations at Parton's theme park, the inevitably named Dollywood, because of her annual Gay Day. ("God tells us not to judge one another, no matter what anyone's sexual preferences are or if they're black, brown or purple. And if someone doesn't believe what I believe, tough shit.")

She's had 41 Top 10 country albums, she has written some of the best loved songs of all time (I Will Always Love You, Jolene), starred in two of the best loved women's movies of all time (9 to 5, Steel Magnolias) and her oeuvre is the backbone of karaoke bars around the world (Jolene for the soloists, Islands in the Stream for the duets). She's the self-dubbed Backwoods Barbie with a penchant for dropping self-mocking aperçus such as: "If I have one more facelift I'll have a beard!" Parton even taught me how to count when I was a child thanks to her Muppet persona, Polly Darton, on Sesame Street, whose signature song was, of course, Counting One to Five.

But while her embonpoint ("Let's say 40DD – at least!") and plastic surgery ("If something is bagging, sagging or dragging, I'll tuck it, suck it or pluck it") are as much a part of pop culture legend as Elizabeth Taylor's multiple marriages, the superhuman quality for which Parton is best known is just how gosh darn nice she is. Film critic Roger Ebert once wrote that meeting Parton made him feel "as if I were being mesmerised by a benevolent power. I left the room in a cloud of good feeling."

"I'm not happy all the time, and I wouldn't want to be because that would make me a shallow person. But I do try to find the good in everybody," Parton says perkily, and later proves it by describing Sylvester Stallone – her co-star in the deservedly little-seen 1984 film Rhinestone – as "just a nut, but so witty!".

Come on, Dolly. You've been so famous and – apologies for crassness – so rich for so long, you must have encountered a lot of venal people with less than good-hearted intentions. How can you still be so sunny and uncynical?

"Oh, I can spot a phoney a mile away," she replies before I finish the question, perkiness instantly replaced with no-messin' toughness. "If people work for me over the years I expect them to be paid what they're owed but I don't expect them to be paid more than they earn."

This sharper tone returns when she talks about the harsh treatment "my little friend Miley Cyrus" has been getting from the press: "We're punishing her for growing up, just like we did to Shirley Temple," she announces sternly. It can be stated with some certainty that this is the first time Miley Cyrus has been compared to Shirley Temple.

But in the main, Parton is as reliably sparkly and hilarious as any of her stage costumes. Her jokes about her bust and her looks are as well-worn yet effective as, say, I Will Always Love You. But it's hard not to suspect that these quips, and the breasts, are distracting, um, fronts – her means of keeping prying eyes away from the private woman within.

Dolly Parton with Lily Tomlin, Jane Fonda and Dabney Coleman in 9 to 5 (1980). Photograph: c.20thC.Fox/Everett / Rex Featur

Parton famously modelled her appearance on a prostitute in the town where she grew up, but she really looks more like a child beauty queen, a girlish fantasy of prettiness. It is as if she settled on her look when she was 10 years old and stuck with it for the next 50 years, knowing that constancy is the safest shield against curiosity. As she herself says when I ask if she minds people always asking about her breasts, "How can people not? They're all you can see!"

One aspect of Parton's life that has not been seen for some time is her husband, Carl Dean. She has been married for 45 years, yet many of her fans have no idea what her husband looks like, or whether he actually exists. The few photos of him in the public domain show a handsome, rather shy-looking man.

"Carl's never wanted to be in the limelight, so I didn't put him there. He wants to be left alone and be a homebody and then hear about what I've been up to when I get back. He's my anchor and I'm his excitement," she says.

But Parton is a master of deflecting one-liners, slaking people's curiosity by leaving them laughing. In her very funny 1994 autobiography, My Life and Other Unfinished Business, she recounts meeting Dean in Nashville at the beginning of her music career and proffers what is surely the 20th century's version of "Reader, I married him" as she explains why she was so keen to marry Dean: "I was getting pretty horny."

Dean's absence from the public eye has led to suspicion among some of her fans that Parton's real partner is her childhood friend and constant tour companion, Judy Ogle.

"Judy and I laugh and laugh when people say such things," she says, not breaking eye contact for a second. "It keeps me humble, having someone around who I know loves me for who I am and not what I've done. Judy and I have known each other since we were itty-bitty girls and we're like Siamese twins because we've grown together and if something happened to her, it would just kill me." Her voice cracks ever so slightly at the end.

In this regard and many others Parton can be compared to Oprah Winfrey, another woman who worked her way out of a deprived childhood in America's south to become a no-surname-necessary superstar, and whose close friendship with her female best friend has led to similar speculation, despite frequent denials.

"Oprah and I have talked about that a lot," Parton agrees. "We share many very special bonds and we have often spoken about how people try to make out that she and Gayle [King] are gay and Judy and I are gay. But we understand that very few people are lucky enough to have a very best friend and keep that best friend all your life."

And like Winfrey, Parton has built an empire out of seeming to wear "my heart and emotions on my sleeve", while simultaneously being supremely professional and self-protective. But just occasionally, that mask slips. At the end of the interview I ask if she is spending the rest of the day doing interviews with other journalists. "Yes," she replies and permits herself the tiniest of eye-rolls before she quickly stops herself and asks if I'd like her PR to take a photo of us together. Obviously, the answer is an enthusiastic yes. A diverting trick it may be, but that trick is irresistible. Her millions didn't come from nothing.

Speaking of those millions, a huge part of Parton's legend is her poverty-stricken backstory and she makes as many references to what she calls her "hillbilly ways" in conversation as she does in her songs. "I'm just a simple silly country girl!" she says more than once. It should perhaps be interjected here that this simple silly country girl owns 15 properties spread out between Tennessee and California. Her management takes great pains to tell me later that Parton prefers to travel by bus than plane when she tours, as though this proves her simplicity. But it transpires that shipping the Dolly buses to Australia for her upcoming tour will cost $1m each. As Parton has said of herself many times, it costs a lot to look that cheap.

Parton was born and raised in the Smokey Mountains in Tennessee, the fourth of 11 surviving children. Her father, a sharecropper, couldn't afford to pay the doctor who delivered her and so gave him a sack of cornmeal, leading to Parton's oft-repeated joke about how she's been "making dough ever since". She began singing on the radio as a child and went to Nashville to start recording and singing songs as soon as she graduated from high school.

"Back in those days people said it was hard for women to get into the business but I always felt being a woman served me well. I had all these brothers and uncles so I understood the nature of men and I didn't go in there feeling all intimidated. I just went in there and said: 'Hey! I have a good product here and we can all make some money here if y'all wanna get involved with it.'"

This self-confidence was to come in handy over the years, such as when she refused to let Elvis Presley sing I Will Always Love You, because his manager Tom Parker was insisting that he got half of the publishing rights. When Jane Fonda approached Parton, who had never acted before, to appear in 9 to 5, Parton agreed – on condition she could write the theme song.

"I was gonna be rich no matter how much it cost/I was gonna win no matter how much I lost," Parton sings in Sacrifice, one of the best songs on her new album.

So what has it cost her? "I gave up time with family and friends, I can't ever plan too far ahead, I didn't have children … " she reels off.

Parton underwent a partial hysterectomy when she was 36 and suffered from serious depression, both before and after the operation: "It lasted about 18 or 20 months. I wasn't doing too well." She credits her recovery to God, work and Sylvester Stallone, and not necessarily in that order: "We were making Rhinestone then and, oh, Stallone was so healthy for me to be around ... "

Time is up and I can have one last question. Tell me one thing about yourself that would surprise your fans, I say.

"Oh lordy, I think they know pretty much everything!" she replies merrily, beginning to get up.

I think they barely know anything.

She looks a little taken aback and sits back down.

"Well actually, that's probably right. They think they know so much but I am very protective and private about my very personal stuff. So yes, they would be surprised by how little they really know me. Ha ha ha!"

• Dolly Parton's new album Better Day is out now; she is touring the UK until 15 September