The movie is Soapdish (which is pretty good, for a silly movie):
For those who haven't seen it, quick recap: Sally Fields' character is a famous and popular soap star. Her niece (actually her daughter) comes to work on her show at the same time that her old boyfriend comes back - the boyfriend who unknowingly fathered the child and was written off the show so he wouldn't mess up the star anymore. What can I say - it's a soap.
Anyhow. Towards the end, the truth comes out because the daughter and the boyfriend are supposed to share an onstage kiss. It's a big, tear-jerker scene, and Sally Fields goes nuts and then breaks down and confesses everything, and everybody's listening... To the nit: A little later, SF is watching a news report of her confession (of course it's on tape; it's a soap). On the tape she says, You can't kiss him. You have to trust me. In the real scene, she never added that last line.