The film is a remake of Christian Carion’s 2017 French film of the same name and, just like in the original, everybody had scripts except for McAvoy, who was given only a brief backstory about his character, meaning his reactions to the film’s shocking twists are genuine responses.
During an appearance on The Graham Norton Show, McAvoy said: "The entire film for me was improvised – the rest of the cast and crew had a script and had all been rehearsing with each other and another actor playing me for weeks.
"I had to learn what was happening next as I went along – it was crazy."
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He continued: "The French director designed the whole thing to preserve the first take, so whatever I did was as real, natural and truthful as possible."
In need of a recap of the film's plot twists and ambiguous ending? Read on for everything you need to know about My Son.
My Son ending explained: Do they find the child?
Following Ethan's disappearance, Edmond visits his ex-wife Joan’s (Claire Foy) new partner Frank (Tom Cullen), and is alarmed when he discovers he has made plans for a new home for himself and Joan which doesn’t have space for Ethan.
Edmond grows increasingly suspicious when he discovers that Frank gave Joan a Valium tablet to help her sleep, accusing Frank of being responsible for his son's disappearance.
However, after beating Frank up, Edmond looks through his phone and spots the same vehicle in two videos of Ethan in the weeks leading up to his disappearance.
Realising this could be a lead to Ethan's location, Edmond tracks down the vehicle’s owner, William, who lives on a farm, and tortures him, forcing him to admit he works for a kidnapping ring and sends them photos of children.
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William explains that the kidnapping ring then selects which children to kidnap – and he and the other individuals working for the ring snatch those children and leave them at Dunmore Lodge to be collected.
Armed with this information, Edmond makes his way to the lodge and finds Ethan there, where he's being watched over by three of the kidnappers.
Edmond leaves a voicemail for Joan, telling her where they are and asking her to contact Inspector Roy (Gary Lewis), who's been investigating the case.
In a dramatic scene, Edmond then rescues Ethan as Joan arrives and the three of them drive away.
Edmond is shot and ends up passing out at the wheel, but Inspector Roy arrives just in time and finds their car, taking the three of them to safety.
Why was Edmond arrested in My Son?
Following the film's dramatic rescue scene, the final shot initially appears to be a perfect ending: Edmond and Joan watch on as Ethan operates a toy drone.
But the scene is permeated by a sense of unease, which only grows as Inspector Roy arrives and it becomes apparent that Edmond has been arrested, though we’re never actually told why.
"I might be a wee while but I am going to come back," Edmond tells his son as he leaves Joan and Ethan playing with the drone and walks to Inspector Roy's car.
One option is that he was arrested for his antics we see in the film: he tortures one of the kidnappers with a blowtorch, breaks into private property and more.
And the other option? Well, in the film, we learn that Edmond travels around the world working for an oil company, and that he's hesitant to hand over his phone as part of the kidnapping investigation, though he eventually does.
Joan later sounds alarmed when she tells Edmond that the the police have raided his offices and taken all the computers.
"They turned the whole place upside-down. They've got the computers. They've got the hard drives," she says, adding that this includes the "Iraq files".
On the upside, Inspector Roy tells Edmond that his help in uncovering the kidnapping ring will likely be taken into account.
"The dismantling of the network is accelerating, you know. I believe that there will be more arrests. It'll be a big list," he tells Edmond in the car as they drive away.
"It could be good for you... The judge will probably take this into account."
Read more:
- David Tennant was the perfect fit as BAFTA Film Awards host – let's hope he's invited back
- Small Things Like These review: Cillian Murphy is superb in subtle drama
Check out more of our Film coverage or visit our TV Guide and Streaming Guide to find out what's on.
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