Cast members of "Buffy the Vampire Slayer," "Gossip Girl" and more are remembering their co-star Michelle Trachtenberg​ after the news of her death.

When the series shifted from the WB to UPN for its sixth season, fans were treated to one of its most innovative episodes: the musical Once More With Feeling. Most prominently for Trachtenberg, whose ballet talents were showcased that week, she opens the episode’s coda, “Where Do We Go From Here?,” singing the opening title line a cappella.

Michelle Trachtenberg, the actor known for “Gossip Girl,” “Buffy the Vampire Slayer” and “Harriet the Spy” died Wednesday. She was 39.