Jow appeared in the first season as Anna, a love interest for Jeremy (Steven R McQueen), but was killed off.
However, in the second season finale Jeremy was able to see her again after he had been brought back from the dead.
"I had absolutely no idea [I'd come back]," Jow told TV Fanatic. "[My friends] were mad at me once the finale aired [because I kept it a secret]."
The Vampire Diaries's executive producer Julie Plec has previously hinted that Anna and Vicki, played by returning cast member Kayla Ewell, have a "master plan".
However, Jow explained that she isn't sure what's coming up for Anna, saying: "[We're] finding things out as we go along."
The Vampire Diaries will return for its third season on September 15 on The CW.
> 'Vampire Diaries' exec hints at Matt struggles
Watch the latest trailer for The Vampire Diaries, which includes a clip from the third season, below: