Assume everyone is a loose passive muppet until they prove otherwise.
Would agree with that, which means you're playing pretty tight and waiting for them to give you their money. So many actions from you depend on what type of player they are that you may get stung unless you know them. Eg. I never normally call all-in with second pair but tonight I'm in a cash game with a drunk guy who is raising pf then betting *every* single flop, so I may think about it.
I play a lot of heads up just now and I do next to nothing in the game until I have my opponent classified. Hard to work out the order but it's roughly:
1) Tricky: yes/no. About 1/20 are, it will be the hardest game, and needs full concentration
2) Donk: yes/no. Continually passing up the button for no good reason, for example, will give him a yes
3) Active/Passive.
4) Tight/loose
5) Aware of how I've classified him yes/no
6) Has he classified me, if so how?
My behaviour will be totally different depening on how these are filled out. Did have a totally new one this evening, who was betting 'colour draws' (eg he holds two clubs and the board comes

Ah, he is betting on another black card coming and him hitting his '5 black card flush'. I guess he was pretty drunk, kept doing it....'
Also, BAD BEATS. This evening, none to report. Zero.