Genre is important not only for movies but for novels too. Too often beginning writers want to throw in everything: "Well, it starts our funny, but then it gets very dramatic around here, but there will also be some dancing and singing."
Although the old adage "make them laugh, make them cry" is true, you need to determine what the major feel of the movie is. If it is a comedy, then every scene has to have comedy in it. That means, not only humorous dialogue but humorous circumstances and action. If you want to study a film that does a very good job of this, take a look at "Talladega Nights". There is funny action and dialogue in every scene. And those scenes that try to be slightly more dramatic, like when Will Ferrel's character is dealing with his flaw, which is fear... Well... This movie is best when it's sticking to the comedy. And even those slightly more dramatic scenes between Will Ferrel's character and his father are still funny.
One movie that does a terrible job of mixing funny with what is supposed to be slightly more emotional is "Mall Cop". Apart from a lot of other problems this movie has (and didn't it do terribly at the box office?), the writers tried to mix what Kevin James does best (being funny!) with scenes in which he is whinning about not having a girlfriend. Bad! They could have found more humorous ways to get that across, instead of trying to appeal with our more emotional sides WHICH DIDN'T WORK!