Amber Benson had the following to say about discovering Tara would die:

“I knew at the beginning of Season Five that Willow was going to go bad and that I was going to be the impetus for that, but I wasn’t 100 percent certain about the dying until the very end of last season. I mean, I wasn’t at all surprised when Joss finally told me when were shooting the season finale last year [The Gift]. He was all excited about it. In fact, I even knew I was going to be in the main credits just for the one episode [Seeing Red]. Joss was like, ‘I want to put you in the credits,’ and I’m like, ‘That’s so evil! How can you do that to the fans?’
“As much as I hate the fact that she died, I think that it was needed. I know they were really worried about making people unhappy, but it was the only way to take Willow to another level. It furthered the plot and made it possible for Willow to go bad. This was the only thing that could have pushed her over the edge. I was all for me being maimed and coming back, but it wouldn’t have been enough. I feel as upset about it as the fans do. I know it was necessary, but still, ‘No, don’t kill her, I love her! She’s part of me!’ I thought I was going to be dead in the 16th episode. They just kept pushing it back, so I didn’t know when I was going to be dying, because they switched around the episodes.”