When a friend sent him a copy of the script for The Dr Blake Mysteries' first episode, McLachlan saw an opportunity to realise both ambitions in the role of Lucien Blake, a GP and police surgeon in 1950s Ballarat who is haunted by the atrocities he witnessed during World War II.
He knew it would be a tough ask to convince the producers he was appropriate for the part, given the character is in his mid-50s and has a gravitas that was the antithesis of McLachlan's public image.
"Sometimes you get that overwhelming feeling that this is one for you and you just gotta go for it."
McLachlan was in LA, appearing in an episode of NCIS: Los Angeles, so while "having a great time shooting some gritty scenes with LL Cool J" he also prepared an unsolicited audition for Dr Blake.
"I already had the beginnings of a beard, with fair bit of grey in it, and I spent a day filming in the Hollywood Hills without any sun protection and squinting as much as I could to highlight the crow's feet around the eyes.
"By the end of that day I was looking a bit worse for wear and the next morning I got a friend to film me doing five scenes and emailed them to the producers."
Although the first season of The Dr Blake Mysteries only started screening on TV One last week, it played in Australia a year ago. "It was one of the top three or four Australian dramas of last year, which was an exciting thing to achieve. Plus, season one recently screened in the UK on BBC1.
"To have an Aussie show picked up by that channel is quite extraordinary but, as I understand it, it also won its timeslot over there and the BBC have bought season two and are already asking questions about season three, which we haven't even shot yet. So yeah, it's been really satisfying for all of us to see our baby do so well."
Its success aside, McLachlan says the series is "creatively the best gig of my career and gives me the chance to work with a whole bunch of wonderful people", name-checking in particular series creator George Adams and NZ's Joel Tobeck, who plays the local police chief and has "become a very close friend who I enjoy working with enormously".
The second season is about to screen in Oz and McLachlan is currently appearing on stage in Brisbane, where he's playing Frank-N-Furter in a 40th-anniversary revival of The Rocky Horror Picture Show.
Although also a doctor, the sweet transvestite from Transylvania is light years away from Lucien Blake, which has caused confusion for some fans of the TV show.
"The other night at the stage door one senior gentleman stood out as the most unlikely Rocky Horror audience member," laughs McLachlan.
"When I asked if he'd enjoyed the show he said, 'Um, well, yes, I think I did. To be perfectly honest, Mr McLachlan, I didn't know what Rocky Horror was.
'Perhaps I should have done my research. When I saw in my local newspaper you were appearing in it, I thought it would be some terrific whodunnit play and, well, let's just say I was somewhat surprised ..."'
The Dr Blake Mysteries screens 8.30pm, Saturdays, on TV One.
