"Tom and I are attached for six weeks. I basically go into the set and do the scene for the lighting guys."
The role involved him doing takes, sometimes alongside other stars of the film.
His first day on the job, he was almost immediately thrust into a practice take with the other actors, including Samuel L. Jackson, Roseanne and Monsters Inc's John Goodman, Room's Brie Larson, and Talladega Nights' John C. Rielly.
"I was actually *****ing myself," he said. "Every big name in the film was in the scene.
"You're hanging on set with them. You just have free range to basically everyone. Everyone was just so chill and so lovely."
Mr Stratton-Smith said there was an "element" of being starstruck when he first saw them, but said that it was important to "play it cool".
But he recalls struggling to keep his cool while working on a film called Unbroken, when his boss sent him to discuss some work with "Ange".
He "didn't click" who he was going to see until he walked into an office and came face-to-face with Angelina Jolie.
"I'm, like, shaking. My palms are sweating."
When Mr Stratton-Smith isn't socialising with the stars, he spends his work hours modelling.
One of his favourite jobs was working on "look books" for clothing brands.
He has modelled for big names such as Prada and Hugo Boss, and has also done commercials.
Mr Stratton-Smith said keeping to a certain size and weight was not too hard for male models.
He would not worry much about it except in the lead-up to an underwear photo shoot.
In that case, he would hit the gym every day in the week leading up to it, and the day before would eat two eggs for breakfast, mushrooms for lunch, and soup for dinner. He would not eat on the day of the shoot.
But after that he would head back to relax at home in "fat pants", watching Friends and eating liquorice.
"If you could see into my home life, it's totally different," he said.
Mr Stratton-Smith said he was "quite good friends" with his neighbours, who would know when he was on one of his diets and would put liquorice in his letterbox the day his diet ended.
While Mr Stratton-Smith isn't modelling or acting, he works 10 hours a week as a tutor at a university, teaching speech and text analysis. He is about to start again as a picture double for Hiddleston, in a different movie.
He has also been involved in Melbourne Fashion Week, which he considers "really fun".
"Out in the back where you don't see anything we're dancing before we go on to the runway. Everyone's naked 24/7, you're running around in your underwear."
He said he would go through three or four different "looks" in about 15 minutes, so had a team of about six people "ripping" his clothes off and putting outfits on.
