Iain was personally chosen by Jim Parsons to play the younger version of his Big Bang Theory character Sheldon Cooper — a wildly gifted but socially awkward young man.
According to TMZ, Iain made $US30,000 ($43,752) per episode in season one of Young Sheldon, with his pay packet bumped to $US31,200 ($45,502) for season two — which is now available on DVD and Blu-ray — meaning the pint-sized star is already a millionaire.
Armitage as child prodigy Sheldon Cooper, in The Big Bang Theory spin-off show Young Sheldon. Photo / Supplied
News.com.au caught up with Iain while he was in Australia earlier this year, with the child star having a momentary blank when asked if he had made any extravagant purchases since making it big in Hollywood.
"I try to stay away from things like splurging, I don't like that, no," he said. "Probably the biggest thing I've bought is a $3 bottle of water."
Despite being one of the most-watched American TV shows of the moment (Young Sheldon's season two finale was watched by a staggering 13.7 million viewers in the US) Iain said he only gets recognised and stopped by fans "occasionally".
"Sometimes, it kind of depends on the city and time and place," he said. "People are very nice about it and I love being part of something that makes people feel so good. So then if I make people happy, that's my job!"
Iain was cast in Young Sheldon after submitting a video of him performing a three page-long monologue that included a William Shatner impression. TV bosses and Big Bang Theory star Parsons were instantly impressed by Iain's talent and called him in for a screen test.
"(The audition monologue was) in one word insane," Iain said of the audition process. "It was hard, but eventually I managed to do it somehow and I pulled through and they asked for a call back and I went and I got it.
"It happened so quickly, it was amazing."
Faced with having to play one of TV's most loved characters of the past decade, Iain prepared for playing Sheldon not by watching the Big Bang Theory but by spending lots of time with Parsons.
"I didn't watch any Big Bang because that's not really appropriate for me," Iain said. "But I did spend some time with Mr Jim and learned a little about Sheldon. Having the world's foremost Sheldon expert to teach the newer Sheldon, it's wonderful."
Armitage, with his Big Little Lies on-screen mother, Shailene Woodley. Photo / Getty Images
Set in 1989-1990, Young Sheldon follows the child genius as he starts high school at age nine.
The budding genius must navigate attending high school alongside his old brother Georgie as well as his relationship with his twin sister Missy, who is still in primary school.
A lot of the show's humour revolves around Sheldon's lack of social awareness and relationship with his family, with Iain's deadpan delivery generating the most laughs — a difficult task sometimes, he admits.
"I'm usually pretty good at having a funny time and then getting ready and focusing. (But) there's one scene where I ask Tam (Ryan Phuong) about surviving psychological torture and he says, 'when my uncle was in the re-education camp he would imagine disembowelling his torturers with a bamboo spike'," Iain recalled.
"And they had really sinister music going on … so many times I was just like (laughing), 'guys, give me a second'."
"They are such talented women that I've gotten to work with in Big Little Lies," Iain said.
"I feel truly honoured to say I've met Miss Meryl Streep and Miss Shailene Woodley and Miss Nicole Kidman, Miss Reese Witherspoon, Miss Laura Dern, Miss Zoe Kravitz … so many wonderful people and I feel so privileged."
Young Sheldon season two is available now on TVNZ OnDemand.
