Watch: I Came By cast and director discuss dark side to Hugh Bonneville
The Downton Abbey star plays Hector Blake in the movie — a retired judge with a track record for assisting refugees in the courts.
But the character might just be hiding something unsavoury, which comes to light when a graffiti activist played by George MacKay breaks into his home.
"When people watch the film, they'll see Hugh in a way that he's never been seen before," MacKay told Yahoo Entertainment UK.
MacKay added: "He's incredibly professional, so he was able to kind of switch that on.
"It wasn't, you know, strange to be around on set or anything but, when action was called and, and that character got switched on, it was amazing to be in the room and suddenly see it happen in front of you."
Bonneville himself said that he was able to step away from the character when cameras weren't rolling, joking with co-star Kelly Macdonald, who plays MacKay's mother.
He said: "When you're dealing with something that sometimes gets into the darker corners of the human psyche, it's important to keep a sense of humour about things.
"So Kelly Macdonald and I really used to just argue about who was going to have the last KitKat in the green room."
As for the character himself, Bonneville said he was reminded of an exchange between acting legends Sybil Thorndike and Peggy Ashcroft, in which Ashcroft was concerned about how to play a character as "horrible" as Lady Macbeth.
"I think Sybil Thorndike said 'well, don't be ridiculous, you love your husband, don't you?'. So that was the key, you know. As long as you understand that there's a humanity somewhere in there," said Bonneville.
He added: "I think [Hector Blake] is a pillar of the establishment. And it was interesting to see as I read the script, as the surprises came along and the twists and turns, that actually he was a little more complex than some of the other pillars of establishment I might have played."
Certainly, for Anvari, part of the joy of casting Bonneville was in having the ability to subvert the persona of such a well-known face to British audiences.
"I'm actually very excited to see how people react to this version of Hugh. He's such a wonderful collaborator and wonderful man," said Anvari.
The Under the Shadow director added: "It was very brave of him to take on this role, and he didn't even flinch or blink at the idea. He went for it.
Read more: Comedians who went dark on the big screen
"I always say that people from a comedy background have this amazing ability to also tap into their dark side, which Hugh did so brilliantly. It's part of the excitement.
"People know him from the Paddington films and his comedies, and of course Downton, so I just can't wait to see how people react to this version of Mr Bonneville."
Bonneville said it was Anvari's vision and his ability to play with expectations around genre that allowed the actor to get to grips with a very different role to what he has become used to.
The star said: "I really needed to talk to him to understand where things were going, and where the background of this character was.
"And I realised that Babak is just dark and twisted as I thought he was."
I Came By is in UK cinemas now, with its Netflix debut due for 31 August.
Watch: Trailer for dark Netflix thriller I Came By