Despite being a Top 40 artist in her own right, Sia remains one of the most enigmatic artists in the pop world, mostly due to her constantly-concealed appearance (wigs on wigs on wigs) and low-key private life.

But the singer-songwriter is ever-so-slightly shedding the mystery in a new interview with Rolling Stone by giving us heaps of fascinating music biz tidbits. Seriously, her industry gossip is unparalleled.

Part of being a songwriter to pop’s elite, Sia explains, is that plenty of the tunes she writes for other artists get rejected or left on the cutting room floor. Instead of abandoning them forever, though, she’s crafted a whole album out of them — This Is Acting arrives Jan. 29 and comprises songs turned down by Beyonce, Rihanna, Adele, Shakira, Katy Perry and more.

One of the most intriguing-sounding songs is “Reaper,” which Sia says she co-wrote with Kanye West for Rihanna’s long-gestating new album. She admits she’s “not emotionally attached” to the song, but she did love another one she intended for RiRi called “Cheap Thrills,” which ended up being “too Brit-pop for her. It’s more Icona Pop.”

"Mostly we've been pitching on Rihanna for the last couple years because she's been looking for a couple years," Sia explained. "Well, it feels like a couple years. It might be one year. They're always looking for that first single.”

She also revealed what it’s like to work alongside Rihanna and Kanye — or, rather, what it’s like to be stood up by them.

“They'll entice me into a session by saying, 'Rihanna will definitely be there' or 'Kanye will definitely be there,' but it's hilarious because I turn up and, almost always, they never come,” Sia said. “So I went into the studio to write for Rihanna with Kanye and neither of them showed up… There were notes from Kanye, and I can't even remember what they were.”

That definitely wasn’t the case with Adele, though. Sia says she and the 25 singer are both alpha types in the studio, which can bring out a certain hardass attitude of hers that makes her feel “insecure.”

“I remember the day after I wrote with Adele. I wrote her saying, 'I'm just writing because I want to make sure I wasn't too overly dominant. I'm feeling a bit insecure that it was a bit annoying working with me as opposed to satisfying,'" she said. “Then she wrote me back and was like, 'What are you talking about?" [Laughs] I was like 'Riiight, OK!’”

On the other hand, things didn’t go quite as swimmingly with Katy Perry (who, by the way, turned down both “Chandelier” and “Elastic Heart,” Sia said).

"She's also quite dominant, and she's extremely analytical. I actually quit within the first hour of our first session,” Sia revealed. “I was like, 'Can we both agree this doesn't work? Like our whole songwriting dynamic?' And she was like, 'I love it. It's like a puzzle to me. It's like a crossword.' And I was like, 'But this is boring for me. The analysis is totally boring for me. It feels like the enemy of creativity.’”

She did add, however, that they did manage to craft a song during their session, and “had a laugh” at the end of it because they were both “able to be authentic.”

That seems to be a fitting anecdote as far as Sia’s concerned. Even though she ended up recycling her outcasts from the Island Of Rejected Pop Songs, it sounds like they’ll be in their purest and most authentic form when sung by Sia herself. Maybe that’s how they were always to be.