And the hardest song to sing during the show is Frankie Valli's 1967 solo hit (written by Gaudio) Can't Take My Eyes Off You.
"It's right near the end of the show and it's a classic song. You don't want to mess around with a classic. It's great fun to sing but it's hard."
Playing Valli in Jersey Boys has to be one of the most demanding roles in musical theatre. Not only does Bilios sing, dance, and act, he goes from playing a 16-year-old boy through to a 70-year-old man. Because as well as telling the Four Seasons' story - from their early days in the 50s as boys from the Mafia-controlled streets of New Jersey to the aftermath of their success - the show is also a poignant document of Valli's colourful and often sad life.
Bilios also wary of making the part his own rather than it being a Frankie Valli karaoke, Stars In Their Eyes-style rip off.
"It's silly to try too hard to be Frankie Valli because no one can. So for me, I grew up performing and I love it, and that is one of the main factors for Frankie too, that he just wants to perform and be on stage. So that's what I bring to it - I love being up there in front of thousands of people just singing my heart out."
He also jokes that his transformation was made complete with the help of the creative team behind the long-running show which debuted on Broadway in 2005.
"That's what they are there for. They've done it 30 times around the world before and so they helped me create that unique [Frankie] sound. It's no easy feat, but it is there now and it sounds incredible."
The role is the 24-year-old Sydney performer's biggest yet - and he rates it as his dream part. "It's better than the top," he laughs.
]"I'm just excited to be doing shows. To be working in musical theatre is hard enough and for me to get a role that is the pinnacle of a frontman in musical theatre is a dream come true."
But it has been hard work. It took six months for him to finally get the part after a serious of tough auditions. Then he was off to New York to see the show on Broadway ("Which was fantastic. And it was my first time in New York as well and it was snowing. The whole thing was amazing.") and work with the creative team.
To hone his best Frankie Valli voice, and learn more about the songs, he then went to Nashville to work with Bob Gaudio, the man behind the Four Seasons' biggest hits.
"It was the Bob Gaudio," says Bilios with a laugh. "I walked in and he was the nicest, genuine, soft, caring and cool guy. And he's produced for every one from Neil Diamond to Michael Jackson. All the biggies. And so then I walk in, and I'm like, 'Oh, God, what's going to happen'. And he was like, 'How ya doing Dion? What's happening? Jump in the studio and let's record some stuff'."
They went through all the songs in the show - including early 60s hits such as Big Girls Don't Cry through to the beautiful Can't Take My Eyes Off You and disco-flavoured Who Loves You - and Gaudio not only gave Bilios musical tips but also the background to the songs.
Though many of these stories are revealed during the show, like Fallen Angel about Valli's daughter Francine dying from a drug overdose, Bilios says Gaudio gave him a real understanding about where the songs came from. "You know, exactly what they were going through when they were writing Big Girls or Dawn, and getting that particular sound. He also really helped me get that Frankie sound as well by getting some extra grunt in there. He knows what he's doing because it's his life, you know."
The Four Seasons were hit-makers but they worked hard for their 175 million record sales - and that's what Bilios loves most about Jersey Boys.
"These guys were from the other side of the tracks, they weren't really educated guys, they all dropped out of school, and between Tommy and Nick I think they did 30 years in prison.
"And there is a line in the show where Tommy talks about their choices - you either get mobbed up, join the army, or become a star. Out of the three you wouldn't think these guys would become stars. And that's why the story - and the show - is so incredible, because it is a true story, there is no artistic licence, they bare their souls."
LOWDOWN
Who: Dion Bilios as Frankie Valli
What: Jersey Boys, The Civic, from April 10
Tickets: From ticketmaster.co.nz and buytickets.co.nz Also see: Frankie Valli (the real one), Vector Arena, April 5
- TimeOut