The Western gave way to the Gangster picture. The Gangster film, which elevated violence to the level of high art, and told stories of the everyman who doesn't take the hard way to fame and money. It is a parable of the New American Dream, a dream in which one can be happy and have all the material things they can take through brute force. And in Gangster films, as in Greek tragedies, the hero is almost always killed or brought to justice by his hubris. William Wellman's The Public Enemy ends with the anti-hero, James Cagney, gunned down, a victim of his own pride and anger.
Musicals in film's history have always been a form of escapist art, letting the audience forget their troubles and go to a place where the everyday is made into a sort of Broadway spectacle, making the juxtaposition between reality and cinema close yet far-away. And since musicals' popularity led to the rise in using sound in motion picture, starting with the hugely popular The Jazz Singer, the world of cinema was changed forever.
Science Fiction as a film genre, started out like the Gangster movies, adapted from dime novels, but quickly became a frontier on which skilled writers could make complex metaphors out of the problems and fears of the current times and put them in a new context, to reach those who would otherwise be uninvolved. In each remake of the Science Fiction classic, Invasion of The Body Snatchers, a different theme is employed. In the original version, which was made during the Red Scare, anti-communist undertones are strongly present throughout. In the remake in the 1980s, the enemy is seen as conformity. A single subject can mirror the times depending on how it is interpreted and what is chosen to be put in the forefront.
Finally the Comedy film has been popular since the birth of film, with Charlie Chaplin's improvised form of slapstick, to Buster Keaton's deadpan in the face of absurdity. Comedies are used most often to make the audience laugh at the frustrations of modern life. But the Comedy has many sub-genres like the Screwball Comedy, which, during the Great Depression, satirized the wealthy and laid the groundwork for the Romantic Comedy, in which the lovers are put to the test and come back together in the end.
Genre's role in cinema has been one that has been fading slightly in the last few years. Films of late have been mixing genres more readily that in the past and it seems that the solid, classifiable genres of the past will give way to new ones, ones that are hard if impossible to identify.