She says it's a phrase that took a bit of getting used to, but, as she found out she had the role just 10 days before filming began, all adjustments had to be made very quickly.
"Normally you like to research, work out what you're going to do with the part and read up," she says, "but there was no time for that, so I just had to read the script and go with my instincts."
On holiday in New Zealand at the time, Julia's since had to catch up on the novels of Agatha Christie, having not read anything by the acclaimed crime author before shooting began on A Pocket Full Of Rye, which was broadcast on ITV1, Sunday August 6.
"Taking over the role has been bloody awful," she says, only half-joking. "Geraldine [McEwan] made a success of the part before me, and Marple is such an iconic role, it's very difficult.
"I think Joan Hickson is considered the definitive Marple now," she says, referring to the late actress who portrayed the amateur detective in the BBC's adaptations of the Eighties and Nineties.
"We're all different in our own ways, and it's important to note Agatha Christie wrote her in two ways, too. At first, she was very much as Geraldine was playing her; frail, Victorian, rather fussy and quirky. She picked up the character again 10 years later and she was a little different - tweedier, sturdier, kinder.
"I can't go in the same direction as Geraldine, I'm not going to do an imitation of her. And any actor will tell you, you have to play certain facets of yourself in any character too.
"It'll be hard on the viewer to accept another physical presence in the role. Hopefully it won't take too many episodes to get to know me and come along my path.
"I'd like to think what we've filmed gets a lot better as we go on," she adds. "Marple has a lot of tricks, you see... all the time making everyone think [she's] actually one step behind them, you know, appearing to be a just an ordinary old woman. It's very hard to do it all."
After Geraldine McEwan decided to step down as the spinster sleuth two years ago, the search for a replacement was on.
Yet Julia's name wasn't on the lips of ardent fans on internet sites discussing their choices for possible candidates. The actress knows, because, she admits, she's checked.
In fact, it was only Agatha Christie's publisher who told her he'd always liked the idea of her playing the role.
"He took a piece of paper out of his pocket and it had the people on there he wanted to play Marple. Geraldine McEwan was at the top, now crossed out, with Julia McKenzie written underneath," she says, proudly. "They're just running though his list, but unfortunately, I didn't see who was third so I don't know who'll take over if I pack up!"
The former Fresh Fields actress shouldn't think too much about packing up just yet. She's signed up for four years of Marple, making four episodes each year.
"It's a big undertaking, but provided I can still walk unaided by that time, I'll be there!" she says, with a glint in her eye.
While she's now 68, it could be said no one really thinks of Julia as an older lady. In many minds, she's crystallised in Fresh Fields and its sequel French Fields, in which she played opposite Anton Rodgers.
"People do give me their seat on the bus now," she says of her age, "and I really don't like it.
"When I'm playing Marple, they do age me a bit, I've gone this colour [pointing to her silvery hair], because I don't like full wigs, so her hair goes into my hair, with the front all crimped.
"They do make me down a bit, but I'm afraid to say her wrinkles, those lines is all natural!
"It's the clothes too, all the tweed and big coats. My husband says, 'Why do you have to look so fat in it?' but it was a very shapeless look back then."
The forthcoming episode is called A Pocket Full Of Rye, and it's one of the most famous Marple stories. The new version is perhaps a little more raunchy than previous takes - Marple is about the only person in it not having an affair - and it boasts an all-star cast.
Helen Baxendale, Kenneth Cranham, Prunella Scales, Rupert Graves, Ralf Little, Lucy Cohu, Life On Mars' Liz White and the late Wendy Richard all pop up in various roles, while Matthew Macfadyen plays the detective investigating a number of suspicious deaths.
"Oh Matthew," says Julia. "Oh yes. He is most lovely, isn't he," she continues, joining a long list of people who don't just admire the ex-Spooks man for his acting skills.
"The cast is incredible. There's a rule no one can be in more than one Marple, but I think that's going to have to change because they'll run out of actors at this rate."
Of all the guest stars, however, there is one who has attracted more interest than any - Joan Collins.
"Everyone I know is asking me about her!" says Julia.
"The minute she was announced as a guest star, I was getting calls from people who'd been in the series before asking if they could come and have lunch on the set to catch up, but only on days Joan was there. Everyone just wanted a look at her.
"She does look amazing. She's older than I am and she's stunning. She is LA, in everything that entails; that's where she exists. Everything is an outfit, everything is a performance and the lipstick goes on every two minutes. That's before you get to the shoulder pads.
"That's the only thing I asked for, when I knew she was coming I asked if I could have bigger shoulder pads than her!
"But she did a very professional job, and she didn't expect to have my Winnebago or anything like that. She wasn't grand or demanding in any way. She enjoyed the experience, but then actors do enjoy being in Marple because it's such good fun.
"That's what I said to my agent a few years ago, 'Get me a part in Marple or Poirot', because I would have loved to have been in one.
"It looks like he went one better."
Julia McKenzie - Extra time
Julia was born in Enfield, Middlesex on February 17, 1941.
Her first major TV role was in late-Seventies comedy Maggie And Her alongside Irene Handl.
She has won or been nominated for many theatre awards, including the Laurence Olivier Award for Best Actress in 1983, which she won for her performance in Guys And Dolls.
She is also filming new episodes of BBC period drama Cranford, alongside Dame Judi Dench and Imelda Staunton.
She has been married to American actor-director Jerry Harte for 37 years.