Where is Madam Rosmerta in the Movie Harry Potter and the Half Blood Prince?

Madam Rosmerta is the kindly purveyor of the Three Broomsticks in Hogsmeade. Although she doesn't play a major role in Harry Potter and the Half Blood Prince, she is mentioned several times throughout the book.

In the most notably comical scene, Hermione rags on Ron for nursing a little crush for the hostess. "And yet I doubt you'd find a woman who sulked for half an hour because Madam Rosmerta didn't laugh at their joke about the hag, the healer, and the Mimbulus mimbletonia."

Yet she also played a role in the plot to assassinate Professor Dumbledore. Neither part of Rosmerta's role is portrayed in the movie, and the story is slightly re-written to accomodate this fact.

In the novel, Madam Rosmerta is placed under the Imperius Curse by Draco Malfoy. This is unknown initially, but he spills the beans in the Astronomy Tower while Dumbledore is talking to him.

Under the power of this curse, Madam Rosmerta is forced to give Katie Bell the cursed necklace. This is indeed the first time that it becomes clear that Malfoy has an accomplice to help him with his mission for Lord Voldemort. Harry suspects that Malfoy is behind the whole necklace affair, but Professor McGonagall is quite adamant about the fact that Malfoy was in detention on the Saturday that the students were in Hogsmeade.

In the movie, this bit is clearly changed. Malfoy is present in the Three Broomsticks, and Harry sees him there. Rosmerta is uninvolved, and Harry has some concrete proof of his suspicions. (Read more about Katie Bell and the Cursed Necklace)

In the book, Dumbledore also surmises that Madam Rosmerta is the one to give the poisoned mead to Professor Slughorn. As Rosmerta has no place in the movie, this no longer makes sense. Although it is not explicitly stated, the assumption is that Malfoy tampered with and poisoned the mead himself. (Read more about Slughorn's poisoned mead))

Finally, Rosmerta returns for the final chapters of the book. When Dumbledore leaves Hogwarts in search of the horcrux, Rosmerta alerts Draco Malfoy. This gives Malfoy time to bring the Death Eaters into Hogwarts through the Vanishing Cabinet, set off the Dark Mark over the Astronomy Tower, and set a trap for Dumbledore.

In the movie, Dumbledore and Harry never go to Hogsmeade. They never see Rosmerta. Indeed, they simply Apparate straight out of the Astronomy Tower and straight back into it. Harry is about to go get Professor Snape when they heart Draco coming up the stairs. It's not really clear how he knew that Dumbledore would be up there in a weakened state. (Read more about Dumbledore and Harry Apparating out of Hogwarts)

In all three cases, Rosmerta has been quietly and simply written out of the story. No doubt the directors found her role unnecessary and time consuming. Unfortunately, this robs the plot of some of its complexity. In the movie, this little side plot is completely consumed by the larger plot of Malfoy attempting to assassinate Malfoy. Alas, that seems to be the fate of a 650 page book turned into a reasonable-length movie.