Why actors deny each and every time?

Have you followed a pattern on how actors are rumored to be acting in a particular film when they in fact are not? Is it the handiwork of a few magazines and newspapers? Or is it the idea of filmmakers to gain publicity before any film is commenced?

A couple of days ago, news reports ran that Ramya Krishnan, one of the best friends of Trisha, with whom she is seen partying around most of the times, has signed her. The film was to be directed by her husband Krishna Vamsi.

Ramya Krishnan was said to be the producer of the film. The once sizzling actress was said to be producing the film. However, when all else remains true, sources close to Trisha has come out denying her acting in the film. While this has been happening for quite some time, it seems a good marketing ploy to promote a film.