Think about the type of comedy you'd like to write. A comedy screenplay can be written around just about any core story idea and combines well with other genres. As long as the focus of the screenplay is on humor, a comedy script can include elements of romance, action and even horror.
Consider specific actors as you write your comedy screenplay. Stand-up comedians often star in comedy movies. Tim Allen, Martin Lawrence and Roseanne are just a few comedians who have become successful in films. Writing with specific comedians in mind can help you focus your screenplay.
Write funny without trying to be funny. A lot of the humor in a comedy screenplay comes from the situation the actors are in and the way they handle that situation. Filling your script with jokes might make a good stand-up comedy routine, but it doesn't necessarily make a good script. Think in terms of funny one-liners in dialogue and visual jokes. The jokes you write should be an extension of the scene.
Write classic characters in your comedy screenplays. Think about the sub-genre in which you're writing. Classic underdog comedies revolve around a protagonist who is generally an underdog with only his likable, if quirky, personality to pull him through. Adam Sandler's movies are good examples of this type of comedy. His characters are usually immature at the beginning of the film and not taken seriously by any of the supporting characters, but by the end of the film he demonstrates his ability to act responsibly without losing any of the core traits that make his character unique.
Add romance to your comedy screenplay. Romantic comedies often include a love triangle and center around two protagonists and their journey to find one another. "Serendipity" with John Cusack is a good example of this type of story line. The humor in the film grows from the mishaps that occur as the two main characters discover their love for one another.
Write over the top. Comedy spoofs are based heavily on hyperbole and visual gags to deliver the laughs. The characters in a comedy spoof are secondary to the action. If a character grows in one of these movies, it's usually by accident or circumstance. The "Airplane" and "Scary Movie" franchises are good examples of this type of comedy screenplay.