1. Selecting a Theme
Imitation may be the sincerest flattery, but it is dangerous for the imitator. And yet to stray too far afield alone is even more hazardous. Successful vaudeville writers are much like a band of Indians marching through an enemy's country—they follow one another in single file, stepping in each other's footprints. In other words, they obey the rules of their craft, but their mental strides, like the Indians' physical footsteps, are individual and distinct.
2. Fundamental Themes
Experience has taught effective writers that certain definite themes are peculiarly adaptable to two-act form and they follow them. But success comes to them not because they stick to certain themes only—they win because they vary these fundamental themes as much as they can and still remain within the limits of proved theatrical success.
(a) The Quarrel Theme. Search my memory as diligently as I may, I cannot now recall a single successful two-act that has not had somewhere in its routine a quarrel, while many of the most successful two-acts I remember have been constructed with a quarrel as their routine motives.
With this observation in mind, re-read "The Art of Flirtation" and you will discover that the biggest laughs precede, arise from, or are followed by quarrels. Weber and Fields in their list of the most humorous business, cite not only mildly quarrelsome actions, but actually hostile and seemingly dangerous acts. The more hostile and the more seemingly dangerous they are, the funnier they are. Run through the Cohan list and you will discover that nearly every bit of business there reported is based on a quarrel, or might easily lead to a fight.
(b) The "Fool" Theme. To quote again from Weber and Fields:
There are two other important items in human nature that we have capitalized along with others to large profit. Human nature, according to the way we analyzed it, is such a curious thing that it will invariably find cause for extreme mirth in seeing some other fellow being made a fool of, no matter who that fellow may be, and in seeing a man betting on a proposition when he cannot possibly win. We figured it out, in the first place, that nothing pleased a man much more than when he saw another man being made to look silly in the eyes of others.
For example, don't you laugh when you observe a dignified looking individual strutting down the street wearing a paper tail that has been pinned to his coat by some mischievous boys? [1]
[1] From the Weber and Fields article already quoted.