Daniel Radcliffe in a scene from Harry Potter and the Half-Blood Prince. (AP)

Los Angeles, July 18 (Reuters): Audiences are voicing snap judgments on movies faster and to more people than ever before on Twitter, and their ability to create a box office hit or a flop is forcing major studios to revamp marketing campaigns.

The stakes are especially high this summer season when big budget movies like Harry Potter and the Half-Blood Prince, which opened on Wednesday, play to a core audience of young, plugged-in moviegoers.

Box office watchers say Twitter, a micro-blogging service that allows anyone to post on-the-fly wisecracks for all the world to see, is the latest weapon in an arsenal of cell phones and computers that audiences use to critique films quickly, often when they are still sitting in theatres. Such word-of-mouth publicity from fan to fan can boost, or bomb, ticket sales.

“Has everything speeded up? The answer is yes,” said Adam Fogelson, Universal’s president of marketing and distribution. “Depending on how big your opening day audience is, word-of-mouth starts playing a factor immediately,” he said.

In recent years those “drops” have widened significantly as communication has speeded up thanks to social networking services like Twitter and Facebook. “If people don't like the movie now on Friday it can die by Saturday,” said Paul Dergarabedian, president of tracking firm Hollywood.com Box Office.