Nicholas Hammond was just 13 years old when he journeyed to Salzburg, Austria with six other kids to film a movie musical about a nun-turned-nanny raising a family in pre-World War Two Austria.

That movie, of course, was The Sound of Music. Today, it is the most successful movie musical of all time. Its fan club knows no bounds in terms of age, nationality, personality or religion.

Fifty years on and Hammond, aka Friedrich von Trapp, is still stumped by it all.

“The question I get asked the most is ‘why is it the most successful movie musical of all time? Why does it continue to grow?’” Hammond tells The New Daily.

“Those of us who were in it have talked about it amongst ourselves many times.

“Honestly, if people knew the answer they’d apply that theory to every movie they made.”

Nicholas Hammond as Friedrich (left) and today.

All Hammond knows for sure is that making the film was a life-defining experience and pretty much a childhood dream.

“My favourite day on set was filming the Do Re Mi scene because it was so much fun,” Hammond reveals. “Bicycles, boats, picnics and running around. It was very active for kids.”

Hammond, who now resides in Australia, also remembers celebrating a milestone on the Salzburg set – an idyllic adventure spot for any teenager.

“I had my 14th birthday on set in Salzburg and I can remember exactly where it was,” Hammond recalls.

“I had a sachertorte and the six other children were at my birthday party in the hotel where we were staying.”

Those “six other children” have become lifelong friends to Hammond and the group catches up regularly.

“Charmian Carr, who played Liesel, is probably my closest friend of the group and I also see a great deal of Kym Karathe, who played Gretel. She’s wonderful and so interesting.”

The group bonded throughout their hectic schedule, sharing several months of singing lessons, followed by soundtrack recording and then days in Salzburg spent between four hours of filming and four hours of schooling.

The Von Trapp kids in Vanity Fair. Hammond is second from the right.

The “kids” recently reunited for a Vanity Fair shoot to pay homage to the film’s anniversary. In the cast photo, they can be seen clutching pieces of those famous curtains, a memento Hammond still owns.

“I’ve got some of the curtains! I’ve also got the Western Union telegram from my agent telling me I got an audition for the part,” he reveals.

“I’ve also held onto the little things Julie has given me over the years – letters and little personal gifts.”

He’s referring, of course, to the legendary Julie Andrews, with whom Hammond still keeps in touch and works with on occasion.

For those of you who are wondering, yes – she really is as wonderful as she seems.

“Having toured with her not long ago when it was just the two of us day in, day out, I got a very close, detailed look once again,” Hammond says.

“Turns out the image I held in my mind all these years was absolutely accurate – she’s generous, she’s kind, she’s funny and she’s deeply talented.”

The entirely loveable cast, featuring the perfectly stern Christopher Plummer, is one of the movie’s main drawcards and possibly the reason a remake has been steadfastly avoided.

“I would counsel anyone to avoid a remake,” Hammond laughs.

What, then, did he think of Lady Gaga’s much-publicised Julie Andrews tribute at this year’s Oscars?

“I had a quick millisecond of being worried knowing her reputation as edgy and unpredictable and thought she might have decided to do something a little bit, I won’t say disrecptful, but maybe a send up or cynical,” Hammond admits.

“But she came out in that ballgown and sang her heart out. I realised underneath all of that exterior there would have been a little girl whose dream was to sing The Sound of Music on stage.”

Lady Gaga isn’t the only one, with Hammond still approached by mega-fans who feel the film changed their lives just like it did his.

“The reaction from the public for virtually my whole life has been so positive,” Hammond explains. “People tell me deeply, deeply personal stories about how the film was very significant to them.”

“You can’t treat it like just another job,” he says sincerely.

“It changed my life.”

To celebrate the 50^th Anniversary of The Sound of Music, the film is being re-released in a 2-Disc Blu-ray edition including the all-new exclusive documentary “The Sound of the City: Julie Andrews Returns to Salzburg”.