Sia’s perhaps best known for writing songs for other people. From Beyonce’s “Pretty Hurts” to Kelly Clarkson’s “Invincible,” her songwriting and production skills are in the highest demand in the pop world. But artists at this echelon are also combing through dozens of songs to include on an album, so some of these inevitably fall by the wayside — witness how a song once intended for Rihanna ended up as Sia’s own biggest hit, “Chandelier.” So she’s taken all those songs that didn’t end up making it on pop stars’ albums and reclaimed them on her own seventh studio album, the aptly titled This Is Acting . You can almost hear Adele’s belt powering through “Alive,” but Sia’s wiry, cracking voice suits the song even better. Power-ballad “Bird Set Free,” also written (along with Tobias Jesso Jr.) for Adele, gains more poignancy in Sia’s own voice, given her public struggles with performance anxiety (“I don't care if I sing off key/I found myself in my melodies”). Though none would deny Sia’s talents as a songwriter capable of writing hits, those looking for more intrigue to her songs are rewarded with tracks like “One Million Bullets,” which possesses both some of her most interesting vocal tics and a powerful chorus, making for one of her best songs yet. In the end, who cares about for whom these songs were intended? Ironically, This Is Acting finds Sia coming into her own as an artist more than ever.