"I feel like I've usually seen them the most in the gym," she told MTV News about touring with the guys. "But they're all so crazy and they're all really fun and they're exactly how they are on camera, just crazy best friends."
The "Now or Never" singer knows so much about the fivesome thanks to her opening slot on their Up All Night Tour last summer. She'll also be opening for the guys on the European leg of their Take Me Home tour, which kicks off Saturday at London's O2 Arena. (Aussie band 5 Seconds of Summer have also been tapped to open for 1D on the tour.) And, she even gave us some insider details on what happens when they work on their toned bods.
"I feel like they do lift weights and they run on the treadmill, but I'm trying to focus on breathing and not passing out on the treadmill myself," she said. "I feel like they're pretty serious about it, a lot of it is good for the voice, especially when you're on the treadmill."
And, she doesn't just know about their fitness regime, the teen singer also knows the guys on a personal level. So, what are they like?
"Well Harry [Styles], Harry I'm always seeing working out everywhere," she said. "Niall [Horan], I always see with a guitar backstage. He's crazy, I love it.
"And then Liam [Payne], I want to say I always see him kind of walking around, wandering around. Then there's Zayn [Malik]. Zayn when I first met he was very quiet and I was a little shocked to see that he was very shy. But it was really cool. And then Louis [Tomlinson]. Louis I don't really see a lot, but when I did see him he was really wild and he was fun and he treated me like I was a normal person, which was great."
So, we had to know, when it comes to pre-show demeanor, who tends to get the most hyped before taking the stage? "Harry is very chill most of the time," she said. "Zayn is kind of a little bit nervous, kind of running around, just very prepared."