Union, who played Zoey, explained that her character's fiancé Demetri (John Cho) was originally going to die as suggested by his flashforward.
"Then I was supposed to avenge his death in season two by representing defendants against the FBI whom I blamed for his death," Union explained.
However, the writers allegedly decided to change the plot because of Cho's popularity and falling ratings.
"Certain storylines got shortchanged," Union said, adding that the show struggled because it is hard "to sell quantum physics when you have the Olympics going on".
However, Union also revealed that she is excited about her role as police officer Gina on the upcoming Army Wives spinoff.
She said: "All the butts I wanted to kick on FlashForward and all of the bad guys I wanted to bring to justice, I'll now get to do as this new character, but with a sense of humour."