A couple other things not mentioned:
1) In Ep. 4, during the Battle of the Death Star, Vader makes the remark, "The Force is strong in this one," as he pursues Luke down the Death Star trench. That was because, Luke was trying to use the Force! Therefore, it opened a channel to Vader's own Force perceptions. "Hmmm, what is this? Some simple X-Wing pilot trying to use the Force! What's up with that?"
2) In Ep. 5, as Luke hangs on for dear life at the bottom of Cloud City, he calls for Leia. Princess Leia, in the Millenium Falcon, suddenly senses Luke is in danger, and she tells Lando to turn the Falcon around, they have to go get Luke. Now how did she get that feeling? That was an early foreshadowing, that there was something more between Luke and Leia than met the eye, and was not simply a woman's intuition!