I'm looking for techniques that are easiest to install and uninstall. That is, a mod makes the change, removing the mod doesn't leave any traces behind. I'm especially interested in being able to add/remove without leaving the cell.

Changing words in existing quest dialog?

Changing conditions for existing dialog?

Adding new dialog to existing quests?

Script fragments attached to quest dialog?

Script fragments attached to objects?

Script fragments attached to actors?