How does the force work in the original trilogy?

[Darth Yan]

one more question? How does falling to the dark side work? Anakin didn't fall immediately after his tusken killing spree and execution of dooku, yet luke is in danger if he kills a helpless vader.

[Eleas]

one more question? How does falling to the dark side work? Anakin didn't fall immediately after his tusken killing spree and execution of dooku, yet luke is in danger if he kills a helpless vader.

I'd say it's more about the conscious decision itself. Anakin didn't fall because deliberate killing wasn't a goal for him. It tainted him and forged the path, yes, but honestly I think it is that Luke knows better. In the presence of the Emperor, fully aware of the magnitude of what he's doing, Luke by killing Vader would (or so I understand it) take that irrevocable step.

[Metahive]

It is not just strong emotions that lead to the Dark Side, they have to come in tandem with an impatient craving for power. As Yoda in ESB says, the Dark Side offers a quick and easy if treacherous way to become very powerful by tapping into one's raw and unbridled emotions. That's how Anakin managed to beat Dooku and what the Emperor hoped to exploit by having Luke beat Vader.

[Ted C]

1.) When luke fights vader on cloud city he doesn't realize vader's there until he starts breathing; Shouldn't Vader's aura give him away? Vader himself sensed Obi Wan's aura at two seperate points in episode 4, and was at pretty much the same range from obi wan that luke was from him. Hell, when Vader tries to sneak up on luke later in the fight, Luke detects him.

Detecting the presence of another Jedi or Sith seems to be imprecise. When Vader sensed Obi-Wan in ANH, he wasn't even sure what it was at first, and even afterward, he only knew that Obi-Wan must be on the station somewhere, but not his exact location; he just figured that Obi-Wan would have to return the the hangar where the Falcon was sooner or later.

At Bespin, Luke is so inexperienced, he might not have the same kind of sensitivity to "Force tremors" as Vader, but a direct threat is something that Jedi and Sith seem much more likely to detect. Even if Vader's aura should give him away, it would only tell Luke that he was somewhere in the city, which is something that Luke should have suspected anyway.

2.) Does the person really have to be actively using the force in order to be detected, cause I have no recollection whatsoever of that happening, and vader fails to notice when Luke Force leaps out of the pit.

Apparently not.

3.) how do force visions work? Palpatine saw visions a few times, yet he fails to see that Luke can become a threat until episode v, when he mentions a "disturbance in the force."

Force visions seem to be pretty vague. Anakin, for example, foresaw Padme dying, but not the circumstances of her death. Palpatine saw Luke as a threat, but not the true nature of the threat.

Furthermore, the Dark Side is reportedly treacherous, so it may "intentionally" withhold some information from someone trying to use it.