Contains major spoilers for Time season 3.

Jimmy McGovern's acclaimed prison drama Time has returned to the BBC for its third – and possibly final – season, and is once again on brutal, gut-wrenching form.

Set in a young offender institute (YOI), we meet a group of young men – kept in line by prison officer Bobby Bailey (David Tennant) and given emotional counsel from chaplain Marie-Louise O'Dell (Siobhan Finneran) – who have erred in violent, sometimes heartbreaking ways: James (Louis McCartney) murdered his disabled brother; Peter (Ollie McNulty) was involved in a violent attack on a stranger.

Certain details about both cases are withheld entirely, or teased out across the three episodes.

But we never learn why Ushindi (which means victory or overcome in Swahili), formerly Christopher, ends up in Alderwick. Perhaps McGovern and his co-writer Samuel Bailey, whose breakthrough play, Shook, is also set in a YOI, didn't want him to be defined by his mistakes, preferring to let his story speak for itself.

What we do know is Ushindi's birth mother Amara, who is Kenyan, sent him to the UK for a better life when he was a young child – a decision that takes on a painful irony given what happens to him.

He was accompanied by his Nigerian father. But he later died and Amara lost touch with her boy.

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We also learn Ushindi has a stammer, which makes his time inside infinitely more challenging, and actor Chukwubuikem Molokwu's preparation considerably more challenging than most.

After auditioning alongside approximately 1,000 other actors and several months of casting, he secured the role.

"It was a bit nerve-wracking," Molokwu told Radio Times. "But I don't think that's a bad thing at all. I think it just shows you care."

Then, the real work could begin.

"It's quite funny because my voice coach Amy Morrison had to work in reverse this time," he continued. "She helps people lose their stammer, but she was training me to get one, so it was a new process for her, too."

It took about a month or so to get it right. Alongside their sessions, Molokwu watched hours of YouTube videos and even put his own stammer to the test in a couple of shops.

"That was crazy and scary, but I wanted to see what it was really like to live with a stammer, off the screen," he said. "What does a real person with a stammer actually go through?"

For one, a full body workout: "I thought it was a facial thing, but I realised that's almost the last thing. There's so much physicality involved. It goes all the way down to your feet and your legs because it's like you're trying to– I don't know if this is the best way to word it, but it's like you want to vomit, you want to get something out from deep inside."

But it wasn't just learning how to stammer. Molokwu also needed to understand how different situations could further impact his speech.

"There are multiple types of stammers: silent, explosive, repetitive," he explained.

"For Christopher, we decided that the more intense he's feeling in a moment, the more intense his stammer will be. And so when he's speaking to someone like Marie-Louise, she's a calm soul, so his stammer would be more relaxed. Versus speaking to someone like Bobby, who's angrier and stricter, so Christopher is going to feel much more tense and the stammer is going to be more explosive."

You see that when he's reunited with Amara and he learns his birth name: "It's a surreal moment. Christopher is a thoughtful character, so I can just imagine him spending all of these years curating this image of his mother in his head – what does she look like? What does she sound like? Is she as kind as I remember? So it's a shock.

"And they don't even speak the same language. She speaks Swahili, he can only speak English, so it's intensified. 'I don't understand her. What is she literally saying?'

"So it’s a lot for him to take. He experiences emotions he can't even understand himself. And he wants to speak, but his body prevents him from doing so."

Ushindi's difficulty speaking with Amara also brought back a memory for Molokwu, who is of Nigerian heritage.

"I remember when I went back to Nigeria when I was six years old [his one and only visit to date, although he hopes to go back] and not being able to communicate with them in Igbo," he said.

"I wasn't taught that when I was a kid but making this did make me think, how great would it be to actually speak to them, in my mother tongue; to be able to communicate with my grandmother."

Watching Ushindi recoil back so viscerally from his mother is desperately sad. But it's what comes later that breaks your heart.

Before Molokwu received his script, he didn’t know Ushindi was going to die.

"That was a surprise," he said of his character's fate, which arrives at the hands of Alderwick's most volatile inmate, Patsy Flynn.

He stabs him in the neck during a reconciliation meeting that Ushindi himself had requested, no less, hoping to speak to his mother face to face rather than through a screen.

How did he feel when he found out? "I was heartbroken, especially after seeing his birth mother for the first time, after finding out about his heritage and where he's from; after these good moments in his life, I want to say for once, and then that happens.

"It's just, it's heartbreaking. It is."

Molokwu was able to spend some time rehearsing that scene, but initially he "wasn't able to get to the level we believed that it should be" before he was given some guidance from Daniel Ryan (prison officer Freddie Jennings).

He also praised his scene partner Louis Shelton for what he was able to tease out of him.

"He does this amazing thing where he can create these really intense, dark eyes," he said. "Sat across from him, he's looking at me like he wants to kill me. It helped so much because I actually felt scared. I felt like I didn't have to do too much there.

"And then it was just thinking about the mechanics of what it would feel like to get attacked like that. There's going to be blood rushing, and failing to get to the brain. So that was an experience, to say the least," – as is watching it play out.

Ushindi's death is one of the most devastating television moments of the year, and an extraordinary – and surely award-winning – turn by Molokwu.

But what about the quieter moments, when he wasn't focused on learning to stammer or on how his body would react after a fatal attack? How did he feel about Ushindi? And did playing Molokwu teach him anything?

"I want to be careful with my words here, but just because somebody has spent time in jail, it doesn't necessarily mean they're a bad person," he said, after some thought. "They've just made some bad choices.

"And in a world where perhaps I didn't get the opportunity to be raised the way I was, could I have lived a life like Ushindi? How many kids are in there because of who they are versus they were just unfortunate to be raised in the circumstances they were raised in, and they had no other way out to survive.

"So my perception on young offenders has definitely changed. The lesson should be to empathise instead of judge on first impression."

Time season 3 is available to watch now on BBC One and iPlayer.

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