This is a prediction/theory that I've just come up about the upcoming "Fluttershy the Pop Star" episode. It seems incomprehensible that somepony who suffers from such crippling shyness should want to perform. Well, here's my idea and, to quote The Element of Laughter herself, "It's a Doozy"!
What am I raving about? In the classic Gene Kelly film Singin' in the Rain, the main female protagonist (played by Debbie Reynolds) is chosen to be the singing voice for a tuneless female star of an upcoming movie (which must be a success of the studio is doomed). The star would lip-synch to the actual singer's music. The problem is that the starlet is so mean and selfish that, at the end, her fellow cast members manipulate her into singing in public and then exposing her voice-stand-in.
I can see this happening in Filli Vanilli: Sweetie is too shy to sing in public, as is Fluttershy. They form a double-act where the beautiful Fluttershy is the front-mare whilst lip-synching to Sweetie's music, who is singing out of sight beind the stage. This music is so fantastic that it actually casts magic on the audience! Fluttershy is on the verge of becoming a global mega-star and even the Princesses are at a concert because they want to see this amazing magic at first hand. Sweetie tries to be nice about it but it's clear that Fluttershy getting the credit is crushing her spirt. Fluttershy isn't willing to gain notoriety from another pony's talent or help destroy her dreams so she arranges for the others Mane Six to expose Sweetie as the real singer before a huge audience and the alicorn princesses.
Sweetie is terrified at first but, when she hears the applause, including the Princesses' clear approval, she gets her Cutie Mark. Sweetie thanks Fluttershy with some simple child's gift that turns out to be the Key of Kindness.
Twilight is so impressed with the magic in Sweetie's voice that she takes her on as her first Personal Student so she can learn to control this Spell-singing. This leads to the events in Twilight Time with Sweetie and her friends exploiting her new status to get their own back on the town's bullies.
I'm fairly sure it's in-character although I'm not sure if it's likely because it would be a big step for the team to let one of the CMCs get their Cutie Mark.