One of the questions I'm most often asked is, "If you could interview anybody in the world, who would you pick?"

It's a difficult question.

When somebody asks it, they imagine that it must be a thrill to meet somebody of whom you are a huge fan.

It is, of course, but it also comes with fear.

What if the person is horrible? What if it's a bad experience?

What if every time you then hear one of their songs, or read one of their books or watch one of their films, it's then a little bit soured by the fact you met them and they were mean or ill-mannered or egotistical?

This happened to me when I interviewed the American author Jonathan Franzen in 2010 about his book, Freedom.

I found him difficult from the first question.

Now every time I consider re-reading The Corrections, one of my favourite books, all I think about is how snippy Jonathan Franzen was.

What if McCartney is a Franzen?

Sorry, this video has expired Paul McCartney rehearsing Day Tripper before his Perth concert

If the question is really, "Of whom are you the most massive fan?", the answer is former Beatle, Paul McCartney.

But do I want to interview him? His work means so much to me, do I really want to risk discovering that he's a Franzen?

This week, I was forced to answer that question. My producer Callum rang me with a mind-blowing offer: McCartney will do his only television interview in Australia with you if you want to do it.

When the offer came, it turned out it really wasn't a dilemma at all. It was a risk that simply had to be taken.

As if any journalist could say no to the offer of an interview with one of the most influential musicians of the past century.

I desperately hoped he would be a lovely person but I also said to myself, "He's achieved so much musically, if he turns out to be awful, and that's the price of being the genius that is Paul McCartney, that's okay, I'm not going to let it affect how I feel about his music."

On the day of the interview, I arrived in Perth — the first stop on his Australian tour — five hours before we were scheduled to head to the stadium.

"You've got plenty of time to have some lunch and then lie down and have a rest," I told myself.

I ate two bites of a sandwich before I couldn't stomach another mouthful and then I spent the two hours before my make-up artist arrived pacing around my hotel room in nervous anticipation, periodically grinning like a lunatic and hugging myself with glee.

I'm going to meet Paul McCartney! I'm going to watch his rehearsal from backstage!

'Why don't you play it?'

Leigh Sales at Paul McCartney's Magical Mystery Tour piano ( ABC News )

As soon as we arrived at the stadium, we met his team (including his right-hand man Stu, who caused the entire ABC crew to audibly gasp when he mentioned that before McCartney, he had been with David Bowie).

In a good sign, everyone we met was warm and friendly. Usually if a team is polite and accommodating, the person at the top is likely to be the same.

McCartney's stage manager, Keith, showed me the collection of guitars with which McCartney tours. ("This is the one he played on the Ed Sullivan show; This is the one they used on Please Please Me").

Then I met his piano technician, who was sitting at the replica Magical Mystery Tour piano (the original is in Sir Paul's house, he told me, but this is the one they've used on tour for 30 years).

"Can I touch it?" I asked him.

"Sure," he said.

I reached over and tentatively played a C chord.

"Remembering your old music lessons?" he asked.

"I can play a little bit," I replied.

"Why don't you play it?" he said, sliding off the piano stool and gesturing that I should take his place.

"No way," I replied, "I can't!" "Yes, you can, have a play," he said.

So I slid onto the stool and played the opening chords of For No One from the album Revolver.

The piano technician smiled; I think because it was obvious then that I was truly a fan, not just somebody going through the motions for an interview.

I played about four bars before I lost it and couldn't play another note correctly, my hands were shaking so hard. This is the piano Paul McCartney plays, I was thinking!

We then went around backstage and Sir Paul arrived to start rehearsals.

I stood in the wings watching as he and the band played Another Girl and then Day Tripper.

When the opening riff for Day Tripper started, I looked at my producer, Callum, and said, "Oh my god," with tears in my eyes.

It was absolutely thrilling to stand in the wings and watch Paul McCartney practise that.

'Oh, luv, give us a kiss'

Paul McCartney hugs Leigh Sales at the end of the interview ( ABC News )

The band finished rehearsing and as I chatted to the crew, I turned around and Sir Paul was about a metre away, walking towards us.

He came and introduced himself and shook everybody's hand.

For almost 60 years, he's had starstruck people standing in front of him, so he's very adept at putting everyone at ease and being warm and charming.

We went on stage and did our interview together.

He was delightful and interesting. He has lovely kind eyes and a gentle way of speaking. I was so happy to see that I was going to walk away with my admiration of him enhanced.

"Sir Paul," I said at the end of the interview.

"I've been very fortunate in my career that I've had the chance to interview world leaders, like Aung San Syu Kii and the Dalai Lama, every celebrity you can think of, Elton John, Patti Smith, but I've never interviewed somebody of whom I'm a bigger fan than you.

"Thank you for all those songs and thank you so much for making time to let us come and talk to you."

"Oh, luv, give us a kiss, come on" he replied and he kissed me on the cheek and gave me a hug.

Every time I listen to a Beatles song now, I'll remember the incredible experience of getting to meet Sir Paul in person and what a beautiful soul he was.

It was one of the greatest experiences of my life.