Why did Tara sabotage Willow's demon-finding spell?

Question

In "Goodbye Iowa", the 14th episode of season 4 of Buffy the Vampire Slayer, before the Scooby Gang finds out about that season's Big Bad they think that Professor Walsh has been killed by a Polgara demon. In one scene:

Willow goes to Tara's dorm room, planning to find the Polgara using a spell that shows nearby demonic activity. However, Tara secretly sabotages the spell and it fails.

As I watched the episode I assumed this would be something that would be followed up on later. I'm at the end of the season now, and this incident hasn't been mentioned.

Why did Tara sabotage Willow's spell? Did the creators of the show originally intend Tara to be an evil character, then change their minds, or did Tara have some unseen motivation for not wanting the spell to succeed?

Answer 1

By this point the writers were setting up some plot points well in advance. Tara's reasons will be addressed in season 5, episode 6 ("Family").

Since the answer is a huge spoiler for Tara, I'm going to allow you a moment to be sure you want to know before continuing.

Sure?

Tara believes she is a demon herself. She sabotages the spell so that Willow doesn't find out, as she's worried Willow won't like her any more once she knows that Tara's a demon. Ironically, she's wrong, as her family have been lying to her. If she'd completed the spell, she'd have learned something she really needed to know.

You ask whether the writers changed their minds. Actually, they did.

Tara was originally going to be a demon. When they came to write it, however, the writers realised they'd already covered this ground pretty thoroughly, and decided it would be more original if she wasn't.

(Incidentally, this isn't the only long-term setup. Things you've already seen in seasons 3 and 4 refer to a character who will be introduced at the start of season 5.)