What are the mechanics behind using надо over нужно or vice versa? What governs when they are interchangeable and when not?
To be perfectly clear: I have no problems simply using whichever sounds most natural in a given situation. So providing just a handful of examples won't do it for me; I can come up with a handful of examples myself. What I'm missing is the big picture; I'm not quite seeing a meaningful pattern.
I know that entire PhD theses have been written on the subject. So while I'm not asking for that level of detail, a comprehensive overview would be nice. And of course if you can actually cite or link to said theses or other papers to back up your points, that would be a major plus.
Edit: so far surprisingly many people have stated things such as "they are absolutely synonymous", "drawing the line between them is unnesessary", and the like. This is simply not true; if it were, this question simply would not exist, and neither would the white papers I hinted at.
There are situations in which надо simply cannot be replaced with нужно. If you can't think of them right now, that's fine; but please do not go ahead and claim they don't exist. As a simple experiment, I dare you to replace every single occurrence of "не надо" in your speech with a "не нужно". See for yourself how much mental effort it will cost you, how long you can keep it up, and how strange it will make you sound. You might pass for an American spy with flying colors, but nobody will mistake you for a native speaker.