A Sketch on Steroids
Good sketches have a beginning, a middle, and an end. They should tell a story. However, you don't have a lot of time to really set up characters or the plot. You have to work fast and just go for the essence of the concept that makes the sketch funny.
Short comedy films work the same way. They are all about the story. You can have lots of jokes or a great surprise ending that wraps up the story in a powerful way, but you need to tell a complete story.
Feature-length comedy movies tell a story, but you have the luxury of time. You can slowly introduce the characters and give the audience a lot of detail about them, as much as they need to know. You can also take your time getting into and resolving the plot.
So, you can look at a short film as a sketch on film and a feature-length film as a big sketch that is made up of a lot of little sketches. You aren't telling just one story over a longer period, but you also need to tell little stories along the way.
If you want to attend expensive film school-quality lectures from the comfort of your own home, check out your favorite comedies on DVD and listen to the commentaries that feature the writers, actors, and directors talking about the film. You'll get an insight into the movie that will show you why decisions were made and how the film ended up funny.
Take a look at Mel Brooks and Gene Wilder's 1974 comedy classic Young Frankenstein. What is the plot? The grandson of the infamous Dr. Frankenstein, after years to trying to clear the family name, inherits his grandfather's castle — and old habits. He creates a monster who terrorizes Transylvania. As a plot goes, it's not very exciting.
Now look at the movie as a series of sketches that advance the plot but work on their own as well. For example:
Dr. Frankenstein and his assistant find a secret revolving bookcase triggered by removing a candle from a holder. The secret door hides a secret passageway that leads to his grandfather's laboratory. However, getting the timing of the mechanism correct causes a lot of problems and ultimately leads to the doctor getting crushed between the bookcase and the wall. (“Put … the candle … back!”)
The Monster meets a kindly blind hermit while he's on the run. The hermit offers him some hot soup but misses the bowl and pours boiling soup onto the Monster's lap. Later, he gives the Monster a cigar, but accidentally lights his thumb on fire, causing the Monster to flee. (“Wait … where are you going? I was gonna make espresso.”)
After trying to refine the Monster, Dr. Frankenstein introduces his “experiment” to a group of doctors and scientists. He and the Monster, dressed in top hats and tails, sing and dance to “Puttin' on the Ritz” (or as the Monster says — “PUDDINN ON DAA RIZZZZZZSHH!”).
The doctor is “nonchalantly” playing darts while being questioned by the one-armed police inspector. His nervousness causes him to misfire every shot with hilarious timing.
These scenes all have a purpose — they are necessary exposition — but they are hilarious. They are classic sketches within a classic film. Each scene is a mini masterpiece.
What makes the movie is the journey, all the scenes and characters we meet along the way. They can be more important than the ending. You need a logical and satisfying ending, but half the time the audience already knows how the movie is going to end: the guy gets the girl, they save the orphanage, etc.