Boyzone star Ronan Keating has called making his West End debut in musical Once "the most liberating experience".
Speaking after his official first night at London's Phoenix theatre, the Irish singer said he was "all over the place" at the start of Tuesday's performance but was "feeling great" by the end.
"The stress and fear when I walked out was overwhelming," he admitted. "But once I got into it, it was wonderful."
Keating will appear in the adaptation of the 2007 film until 21 March.
The 37-year-old plays a character known simply as 'Guy' in this romantic tale of a Dublin busker who meets and makes music with a young Czech mother.
Keating, who has enjoyed success as a solo artist as well as with his Boyzone bandmates, said appearing in Once was "a very different monster".
"You're very exposed on stage," he said. "I've done a week in the show, and every single night you walk out feeling naked."
The singer said he had not considered musical theatre before being offered the show, regarding it as the domain of his late bandmate Stephen Gately.
Gately, who died in 2009 at the age of 33, combined his pop career with roles in West End musicals including Joseph and the Amazing Technicolor Dreamcoat and Chitty Chitty Bang Bang.
'Play with songs'
"I never thought I'd do a musical; that was Steo's gig," he said. "That was his thing and he was amazing at it, but it wasn't for me.
"It was this show that changed everything. It's more like a play with great songs, so it was something I thought I could do."
Keating, who juggled rehearsals with his judging duties on the Australian version of The X Factor, said he was paying tribute to Gately by wearing red socks every night on stage.
"Red was his favourite colour so that's my little tipping of the cap to Stephen," he revealed. "I wish he was here."
Keith Duffy, Shane Lynch and Mikey Graham of Boyzone were in the audience on Tuesday to cheer the chart-topping band's lead singer.
Broadcasters Fiona Bruce, Mariella Frostrup and Julia Bradbury were also in attendance, as were actresses Claire Sweeney, Maureen Lipman and Michelle Collins.
Writing in the Evening Standard, reviewer Fiona Mountford said Keating "impresses" and gives "an extremely creditable account of himself" in the musical.
John Tiffany's "thoughtful production", she continued, "exquisitely" maintains the "fragile equilibrium" between Keating's 'Guy' and Jill Winternitz's 'Girl'.
"It feels patronising to boyband singers-turned-actors everywhere to say he is surprisingly good," said David Lister of Keating in The Independent.
"So let's rather say that he is highly effective in exuding a careworn melancholy that disguises loss. He captures the hearts of the audience seemingly effortlessly."
Glen Hansard and Marketa Irglova played the lead roles in the 2007 original and went on to win an Oscar for their song Falling Slowly.
The stage version opened in New York in 2011, transferred to Broadway in 2012 and won eight Tony awards before its London launch last year.