Arrowverse's Monitor actor originally thought he was auditioning to play Superman villain General Zod

"They prepared sides," he told RadioTimes.com of his audition. "It was a scene with Superman and the character breakdown was 'A powerful Kryptonian with godlike powers' - I thought it was about Zod. Once they said 'Krypton', it just made me think of 'Who's left that's from Krypton?' and also a powerful being like that?

"They did everything to throw you off, that you wouldn't think it was the Monitor or anyone of that nature. But it was someone very strong, I knew that much."

Garrett eventually discovered who he'd be playing "after I got the job and right before I was about to take off to Vancouver for [costume] fittings. I was like, 'Wait, who am I playing?!' – I asked my manager to find out and they were like, 'Oh yeah, we forgot to tell him, we're sorry!'.

"Once I found out, I was over the moon. But yeah, they held that close to the vest until the last minute!"

But that wasn't the last surprise to come Garrett's way, as he later discovered he'd be playing not just the Monitor but also his villainous counterpart the Anti-Monitor.

"I didn't know till maybe the third or fourth episode I did of Arrow this year," he recalled. "And then they said it so casually, in passing, like 'Yeah, have you seen the specs for the Anti-Monitor?'.

"I'm like, 'No, I haven't... I don't know why anyone would show me!' I was like, 'Who've you guys cast for it?' and they said, 'Yeah, you're playing the Anti-Monitor'. It was like, wait, what? Like, run that back again? They said it like everybody knew but me!"

Both characters will play a major part in the Arrowverse's upcoming Crisis on Infinite Earths crossover, an epic intergalactic adventure based on the 1985-86 comic series.

"Originally I thought I was doing three or four episodes and then it turned out to be a lot more," Garrett explained, with his new contract not effectively making him a series regular across all of the Arrowverse shows. "I'm at the front of the cast order with the rest of the cast, but it's not a series regular in the traditional sense.

"I'm not attached to any one show, I'm more of a Crisis series regular, a universe series regular. They've never really done a contract like that – but it's been amazing!"

For Garrett, it's all a wonderful pay-off to a long journey, with the actor having previously auditioned "for a handful of parts" across the Arrowverse over the years.

"I got close a handful of times, for all the shows, from Supergirl to Arrow to Flash, and it didn't work out, or they wanted me but the schedules didn't work. So it all happens for a reason, I guess!"

In the US, Crisis on Infinite Earths will air on The CW, beginning with Supergirl (Sunday, 8th December) and continuing on Batwoman (Monday, 9th December) and The Flash (Tuesday, 10th December). The crossover will conclude after a midseason break, with episodes of Arrow and Legends of Tomorrow (both Tuesday, 14th January).

Crisis on Infinite Earths will air on Sky One in the UK at a later date.