"I suppose you want to listen to some music?" he said, slipping into a brilliantly terrible James Corden accent to add, "Come on then, Niall, let's listen to some music."
This isn't Niall's first time helping James, either. Back in 2015, when he was still a part of One Direction, the entire band hopped in the ride for one of Corden's earliest installments. A lot has changed for both men throughout their long friendship, which could explain why James felt the need to put that friendship to the test with a lie detector test.
The first thing he asked got a big reaction from the crowd when Niall answered "yes" to the question of if he thinks One Direction will ever get back together. That's huge news for 1D fans. And it's something Niall will probably approach with a whole new attitude. They all just might.
Earlier in the ride, and while looking at some of the most ridiculous 1D merch, like toothpaste and action figure, Niall talked about what being away from the group has given him and it's a whole new perspective.
"Having looked at 1D from the outside now, I got to see how big it was," he said. "'Cause when you're in the band, your work just becomes a bit normal, like you're in a little bubble and you don't get to see what it looks like from the outside."
"It's only looking back now I realize how lucky we were," he continued. "There was no one doing stadiums at the time. It's just insane."
He actually got a little emotional during a silly little exercise where his 26-year-old self talked to his 16-year-old self via that 1D doll, laying out the amazing things that were yet to come in his life. It was a sweet moment, with both men surprising themselves with their emotions.
But all of that brotherly love struggled to hold up to the test when some harsh truths came out. When asked his favorite late-night show to perform on, Niall tried to lie that it was James', but the detector accepts no lies. The real answer? "Sorry man, it's 'SNL.' 'Life from New York' and all that."
That was nothing compared to James' even more brutal response. First, he confessed that he doesn't like Niall's darker hair, but then he was asked which song he liked better: Niall Horan's "Slow Hands" or Harry Styles' "Sign of the Times."
While he took a long time to answer, to his credit, James did not try to lie. "I think 'Sing of the Times' is a better song," he ultimately confessed. But it wasn't all bad news, as the detector also confirmed that Niall was has favorite member of 1D, so take that Harry!
Easily the funniest moment of the whole video came when James decided it was time for Niall to face one of his biggest fears, which inexplicably is pigeons.
"I do enjoy a good walk in the park, but if any pigeons come into it-- I think they're horrible," Niall said. "I don't like the fact that they're not scared of us."
Well, what better way to deal with that than to have not one, not two, but three pigeons on your body. Niall was visibly uncomfortable, admitting that he was shaking. "This is the worst day of my life and it's all thanks to James Corden," he declared.
The cutest story to come out of the whole thing, though, had to be the absolutely adorable way that Kim Kardashian explained who he was to her daughter North when the two of them ran into him backstage at an Ariana Grande concert.
"This is Niall, he's the guy who sings from your toothbrush," Kim told North, according to Niall. That One Direction merchandise still saving teeth from gingivitis one song at a time!
Check out the full video above to see what Niall really thinks of Simon Cowell, and the absolutely cringe-worthy email address he used to have -- and actually used when he auditioned for "The X-Factor."
"How did they even take me serious?" he asked.
"How did anybody take you seriously with that as an email?" James countered.