Joss Whedon also wrote the 1992 "Buffy" film. Because of the dramatic differences between his screenplay and the finished film, he doesn't consider it canon as established by the TV series. In fact, the series isn't based on the completed movie at all: it's based instead on the screenplay Whedon initially wrote.
Willow's conversation with a dead person in this episode is with Cassie, of which it is explained that Willow isn't allowed to see Tara because she killed people. In actual fact, the writers did intend for Tara to appear in this episode, however actress Amber Benson was busy with other filming commitments.
While Buffy is the universe's primary focus, Angel holds more appearances in Buffyverse canon, appearing in 168 episodes of Buffy/Angel (as well as the canonical comic book "Angel: After the Fall"), as opposed to Buffy, who only appears in 149 episodes and the canonical comic book "Buffy Season Eight."
Many gay and lesbian rights groups were angered by the decision to kill Tara. Joss Whedon has argued that the decision was made not because Tara was gay, but because she was involved with Willow; had Willow remained with Oz, he would have been killed instead. Killing Tara was a way for the writers to advance Willow's storyline and transform her into Dark Willow for the season finale.