Michael Kenneth Williams used jailed family members as inspiration for The Night Of ...

If you thought Michael Kenneth Williams' standout performance in The Night Of ... seemed realistic, there's a reason for that.

Williams, who plays hardened prison gangster Freddy Knight in the hit HBO crime show, based his brooding performance on his own family members doing time in jail.

"I know that situation all too well, unfortunately, due to my nephew and some other family members who have been incarcerated," Williams says.

"They're no angels. They've done the crime."

Williams, 49, is scoring major accolades for his role in The Night Of ..., an eight-part murder mystery about Nasir Khan (played by Riz Ahmed), a young student who wakes up after a one-night stand to find himself the main suspect in a complicated murder case.

Read more: Karl Puschmann: The Night Of is quality beyond all reasonable doubt

It's not Williams' first role as a criminal in an HBO show, after his stand out performance as moralistic street gangster Omar Little on The Wire gave the character cult status. Even US President Barack Obama is a fan.

In The Night Of..., Freddy helps Khan survive his first weeks in prison, and their bond soon grows into something more sinister as they begin grooming younger inmates to bring drugs into prison for them.

Williams says the trip to work on the show each day reminded him of visiting family members in prison.

"(On) the ride up to the set going through Manhattan, through the Bronx, through Yonkers ... that's the route ... to get in to go to jail, to be put in prison," he says.

"I was like, 'Wow, that's creepy'. And then you get to the set, and it's just as creepy. You felt like you were in a prison yard and it was just as dark grey."

The show goes to great lengths to detail prison life, from the gang alliances that form to constant outbreaks of violence and the survival tactics inmates use to cope.

Williams says the role helped him feel closer to those he knows in jail, and gave him a better understanding of what they were going through.

"I got a glimpse of what they deal with on a daily basis. It made me more empathetic to the situation - and it made me more proud of them. Because my nephew, he's been incarcerated now some 20 years (without) incident, not one bad stain on his record.

"He mentors young men in there, whether it's a Scared Straight program or HIV-AIDS program. He's always positive. He got married in there. Got his education in there.

"So (playing) Freddy gave me an insight into all these things I see and these beautiful characteristics I see in my nephew."

Despite playing Freddy with a brooding intensity that makes you believe he really is a longterm inmate, Williams says he wouldn't be able to cope with prison life himself.

"I've never done time in prison, thank God, because I don't think I would have the balls, I would have the stamina to survive that."

And as much as he loves the show, he's glad his time as Freddy on The Night Of ... is about to come to an end.

"Ending this show was the most satisfying (part)," he says. "We shot it in a very brutal winter. It was a hard, hard shoot. (There was) nothing warm (and) fuzzy about this shoot at all. It was so dark."

* The Night Of's 90-minute finale screens on SoHo tonight from 8.30pm. The show is also available to stream via Neon.