McPhee was an early fan of the show, which unravels the mystery of a 16-year-old girl named Bree in short video segments posted on YouTube, Revver and an official Web site.
"I followed it from the beginning," McPhee told The Associated Press. "I thought it would be fun to meet all those characters."
McPhee's management contacted the creators of the Web series about a guest spot. The writers had already been devising a female character who briefly meets Bree and had been searching for ways to cast fans as characters. They readily agreed to cast McPhee in the nonrecurring role.
"She has an album coming out and thought it would be fun to go on the show," said "Lonelygirl15" co-creator Miles Beckett.
A song from McPhee's debut album, which hits stores Jan. 30, will be featured as background music on Friday's Webisode, although McPhee will not be seen singing.
Instead, the 22-year-old will play an unnamed character who meets Bree and her friends after they briefly emerge from hiding. The episode, titled "Truth or Dare," was filmed last week. McPhee did not get paid for her role.
McPhee said she enjoyed the experience, especially the natural way she was able to deliver her lines. The show, though scripted, is shot in the form of a journal, where characters speak off the cuff into a Web camera.
McPhee said she learned some upcoming twists and turns of the closely guarded plot, but laughed when asked to share them.
"No!" she said, adding she would love to reprise her role.
"I would definitely go back if they asked me," she said.