I had a small revelation about writing the other day. Some friends and I were larking about on Twitter with Monty Python references. Whatever you may or may not think about them, Monty Python made an indelible mark in TV comedy in general and British comedy in particular, much like The Goon Show did a generation earlier on radio.
But there was one thing that was pertinent to the conversation. And that is the incredibly fast paced, well-written dialogue. Both the Monty Python sketched and The Goon Show, as well as most successful sketch comedy written since, requires very fast characterisation. Basically, you have to be able to tell the characters apart, and strongly so, within the first few lines.