Sarcasm is an adjusted stress reaction.

How do you come up with things like this?

*grin* You really want to know? It's not so much a process as a pattern:

1. Have random amusing thought. File it away for possible use as one-line joke in LJ.

2. Suddenly find dialogue and possibly situation based on idea in head. Where did it come from? The idea has simply grown.

3. Dialogue keeps growing.

4. Write down dialogue, filling in gaps with exposition that suits the scenes.

It sounds less than magical put down like that, but this is the way it usually goes with comedic fics. Dramatic fics follow much the same pattern, but are more likely to have scenes as well as dialogue appearing apparently out of nowhere.