Trivia: There were a number of takes for the scene in which Jabba the Hutt captures Leia. Because the puppeteers working inside Jabba could not see Carrie Fisher properly, the operator for his tongue accidentally "licked" Carrie's face during several of these takes. Thus in the final footage, Leia turns away from Jabba in disgust the moment his tongue appears.
Trivia: The Rancor at Jabba's Palace was voiced by a Dachshund.
Trivia: Co-producer Robert Watts makes a cameo as the driver thrown out of the Scout Walker by Chewbacca during the battle of Endor.
Trivia: A couple of German Star Wars fans have - by use of acting, Star Wars props and computer graphics - filmed a story about how the shuttle Tydirium the rebels use to get to Endor was captured in the first place. The story can be found under the title "Tydirium - The True Story".
Trivia: Alan Webb was originally cast in the role of The Emperor, but had to withdraw due to illness.
Trivia: The scene in which an enraged Luke calls his lightsaber into his hand was shot "backwards" - Mark Hamill made himself look upset, entered the shot as if leaving, looked calm, then threw his weapon away. Then the shot was run in reverse.
Trivia: Boba Fett does not have a single line of dialogue in this film. The only sounds he makes are screams when he falls into the Sarlacc pit.
Trivia: Before the Endor Space Battle, one of the controllers says to Admiral Ackbar that there are enemy ships in Section 47, to which Ackbar responds, "It's a trap." In the original script, the character said, "It's a trick!", but a test audience did not respond well to it, so they amended the line.
Trivia: The Jabba the Hutt puppet required four tonnes of clay to sculpt and was animated by three puppeteers on the inside.
Trivia: During story brainstorming sessions, writer Lawrence Kasdan suggested a radical new ending for the film: Luke fools the Emperor into believing he's joined the dark side, puts on Darth Vader's helmet, mans the Death Star controls at the Emperor's behest so that Luke himself can destroy the rebel fleet... but instead Luke proceeds to blow up the Emperor's home planet! It's unclear just how serious Kasdan's suggestion was, but George Lucas vetoed it rather quickly.
Trivia: For those wondering how Han Solo's proposed death (referenced in other trivia entries) would have gone: instead of Leia getting shot in the arm by a stormtrooper in the climactic battle on Endor, it would have been Han getting shot in the chest. The rest of the movie would run almost identically - same dialogue, everything - only Han quietly succumbs to his injuries after the Death Star blows, and he and Leia share a final kiss.
Trivia: Though the dialogue is mostly the same, the script describes Luke as being much more upset and confrontational in his scene with Obi-Wan than how Mark Hamill plays it in the film. Luke is especially appalled at Obi-Wan's "from a certain point of view" explanation and quite literally turns his back on his mentor. Luke does eventually calm down, and is so fascinated by Obi-Wan recounting the story of his father's fate (the script goes into a bit more detail) he mostly forgets his earlier disgust.
Answer: Darth: "Obi-Wan once thought as you do. You don't know the power of the dark side. I must obey my master." It's implied that the dark side is intoxicating, once you totally give in to the dark side it has a hold on you, and appeals to morality and rationalization are useless against your lust for its power. (On a symbolic level, the dark side is a metaphor for vice. Darth Vader is an addict and abusive parent. It's actually funny how many scenes still make sense if you replace "the dark side" with "the bottle" or similar).
TonyPH
If we bring the prequels into it, it's one of the criticisms of those films that they only make the question of how much Anakin is a "true believer" more confusing. But it stands to reason that at first Anakin may feel vindicated in his resentment toward the Jedi. Later on, Vader may not feel that as strongly, but by then his anger has turned toward himself for failing to save Padme. He may feel that a man as terrible as he does not deserve to be "rescued" from the dark side, leading to a feedback loop where he only gets further enamored with its power and does more evil things which causes him to hate himself even more, and so it goes.
TonyPH