He said he was simply "messing around" on the piano when Swift overheard him singing the first verse of "Exile."
"It was completely off the cuff, an accident," Alwyn told Vulture. "She said, 'Can we try and sit down and get to the end together?' And so we did. It was as basic as some people made sourdough."
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He also revealed that he wrote the fully-formed chorus of "Betty" after he "probably had a drink and was just stumbling around the house."
"We couldn't decide on a film to watch that night, and she was like, 'Do you want to try and finish writing that song you were singing earlier?' And so we got a guitar and did that," he explained.
Alwyn added: "Who doesn't walk around the house singing?"
When pressed, Alwyn said he wasn't trying to write songs in Swift's style, but was inspired by The National. The band's own Aaron Dessner is credited as the lead producer on "Folklore."
"The truth is, if I had a pound coin for every time someone told me I've been engaged or I'm getting engaged, I would have a lot of pound coins," he told Vulture. "If the answer was yes, I wouldn't say. If the answer is no, I wouldn't say."
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