They spend every day together as Sophie and her love interest, Sky, in the musical Mamma Mia!, but they have yet to tire of each other's company.

In fact, Lucy May Barker, 26, and Phillip Ryan, 28, are a couple off-stage as well, after meeting at the auditions for the musical in 2015. They are now engaged and plan to get married in 2020.

The chemistry and adoration between them is palpable, with each holding the other's hands and finishing each other's sentences during an interview with The Straits Times last Wednesday at the lobby of Sands Theatre at Marina Bay Sands, where they have been performing since Nov 3.

"There are plenty of people on this tour who have spent months away from their partners," says Barker, who is British.

"But we've got to enjoy it for what it is now because we could be doing two different jobs and trying to see each other in different time zones," adds Ryan, who is Scottish.

Audiences can see the real-life chemistry of the pair on stage at Mamma Mia!, which is playing now at the Sands Theatre at Marina Bay Sands until Nov 18. The musical was last in Singapore in 2014.

The brainchild of producer Judy Craymer, Mamma Mia! combines a story of love, family and friendship, all set to the classic hits by Swedish pop group Abba.

Barker's character, Sophie, is on a quest to find the father she never knew, while her mother Donna (played by Shona White) has to face three men from past romances.

The feel-good musical romp is also one of the longest-running Broadway shows and was made into a hit movie with the same name starring Amanda Seyfried and Meryl Streep in 2008 and again this year, in the sequel Mamma Mia! Here We Go Again. Next year marks the 20th anniversary of the musical.

In a serendipitous turn, Barker and Ryan grew up listening to Abba.

Barker remembers listening to the group's greatest hits compilation, Abba Gold, on her CD Walkman while walking her dog, while Ryan says his father had a jukebox in their home, "where every second record was an Abba one".

"Including in rehearsals, we've sung these songs over 1,000 times," says Barker. "If someone had told me at age eight or nine that I'd be singing those songs, I would never have believed him."

Having been part of the travelling musical for the past three years, the couple will be retiring from the roles in January next year when the musical ends its run in Hong Kong.

"I play a 20-year-old, but I'm 26, so there's a slight expiry date on me playing Sophie," quips Barker. "But I guess we're ready to do new things."

"This is my only big musical career-wise, so I'd like to see what else is out there," adds Ryan. "In any case, it never leaves you - you hear the first note of a song and immediately you know, 'That's Abba'."