Trivia: In the movie, Jeremy Bulloch, who plays Boba Fett, is also seen out of his mask; namely as the Imperial officer who brings Lando, Leia, Chewie, and C-3PO to Vader's shuttle.
Trivia: In the 1996 novel "Shadows of the Empire," it is revealed that Dash Rendar was one of the snowspeeder pilots during the Battle of Hoth.
Trivia: There are two additional scenes that are not included in the special edition. The first one is near the beginning when Han is asking if Luke has made it back, you will notice that there is a slaughtered Taun-Taun on the ground. In the scene they discuss that something has killed it. Later, the Ice Creatures attack the Hoth base and get locked in a storeroom. A sticker is placed on the door saying do not open. In some of the promotional materials there is a shot of C3PO peeling off the sticker. Stormtroopers open the door and get killed. When Vader enters the base, he asks 'What killed these men?' After all, they had to blast into the room, and it's already full of dead Stormtroopers.
Trivia: When Darth Vader is addressing the bounty hunters, next to Boba Fett is IG-88, the bounty hunter from "Shadows of the Empire".
Trivia: THX-1138 is referenced when, on Hoth, General Rieeken says "Send Rogues Ten and Eleven to station three-eight."
Trivia: In the version released in 1980, Emperor Palpatine was voiced by Clive Revill, but was played by an old woman with makeup and a fake monkey eyebrow bump. In the 2004 DVD (and in "Return of the Jedi"), he is voiced and played by Ian McDiarmid, who played the Emperor in later films.
Trivia: A special effects technician added a tennis shoe to the asteroid scene. It can be seen flying from the central-left side of the screen to the lower right. It's right at the beginning of the exterior shot after Han Solo announces that he's going for one of the larger ones, and Leia, C3PO, and Chewbacca voice their objections. (00:39:40)
Trivia: Right before the first transport is away, there is a shot of Leia and others in the control room on Hoth. During this scene a clicking and screeching sound can be heard. It sounds like a clock ticking. This is a recording of a shortwave radio station CHU from Canada, complete with the second ticks.
Trivia: John Morton has two roles in this film. Not only does he play Dak (Luke's gunner) during the Battle of Hoth, he also plays Boba Fett in the sequence when Fett confronts Darth Vader in the Bespin hallway during Han Solo's torture, as Jeremy Bulloch was unavailable.
Trivia: Jeremy Bulloch (Boba Fett) is the half-brother of Robert Watts, the associate producer of the movie.
Trivia: Lobot's name is never mentioned in the film, and in the end credits he is simply listed as "Lando's Aide."
Trivia: The name "AT-AT" is never spoken in the film. Instead, the machines are only referred to as "Imperial walkers."
Trivia: When a Rebel soldier is in a turret is destroyed by an Imperial walker during the battle of Hoth, a "Wilhelm scream" can be heard.
Trivia: When Chewbacca throws an Imperial stormtrooper off the ledge before Han is put into carbon freeze, a "Wilhelm scream" can be heard.
Trivia: The facility of Cloud City is never referred to by name in the film..
Trivia: In Cloud City, when Chewy finds C-3PO in pieces in what looks like a droid scrap room, you can see what looks like the upper half of IG-88 leaning up against the wall. It is revealed later in books to be the IG-88 unit that went in search of Han, but was intercepted and destroyed by Bobba Fett.
Answer: The short, short answer to this is "Yes... from a certain point of view." The long answer is complicated and depends completely on what timeframe you mean by "always." If you're going back all the way to the early rough drafts of the early-mid 70s (which actually resemble Episode I more than they do the Star Wars of 1977), you'll find there's a cyborg father figure protagonist that makes a heroic sacrifice, and then another character that is a "black knight" villain that eventually turns to the side of good near the end. Just to make things more complicated, there is yet another character, a villain by the name of "Darth Vader" that is a human Imperial officer like Grand Moff Tarkin. It may be a stretch to count all that as "Darth Vader was always the father" but the pieces were all there, at least.
TonyPH
(1) Now the earliest explicit mention on any documented material that Darth Vader is Luke's father comes from notes Lucas made outlining the general story of the trilogy and its place in the larger Star Wars saga. These were found in the archives for The Empire Strikes Back, but they are undated and we don't know if they were written before Star Wars (1977) and carried forward, or if they were written afterward. These were found fairly recently (made public in 2010) and as far as I know Lucas has never commented publicly about them.
TonyPH
(3) One thing we know, at least, is that Lucas had come up with the idea of Darth Vader the father before starting work on The Empire Strikes Back. Something incredibly odd, though, is that the first draft written by Leigh Brackett does not feature the twist (and in fact introduces Anakin himself as a ghost); for a long time many fans took this as proof that Lucas hadn't thought of the idea at all by then, but after the series outline was discovered it was made apparent that Lucas simply hadn't told Brackett for some reason. Perhaps he wasn't sure yet that he wanted to go through with it, or maybe at that point he was thinking of revealing it in the third film. Either way, Lucas would write the second draft himself, and that's where the twist first appears in script form.
TonyPH
(2) Something that must be understood about Star Wars (1977) is that it was an ALTERNATIVE to his original plans of a saga. By then he didn't think it was realistic that he would be able to make a long series of many movies, so he came up with a "Plan B": he crammed the general story of the trilogy into one movie. So we know that when Star Wars (1977) was filming, Darth Vader was NOT Luke's father, because this one movie was IT, that was the whole story. But what we DON'T know, is whether that means Lucas had abandoned the idea of Vader being the father in order to simplify the story, or if Lucas simply hadn't thought of that at all just yet.
TonyPH
(2, cont.) On a side note, you can tell by watching Star Wars (1977) how it has condensed the story of the trilogy. The middle portion has the characters trying to escape capture from the Empire while one of them loses a duel with Darth Vader (like The Empire Strikes Back) and the third act is a final battle against the Death Star above a forest moon (like Return of the Jedi). The first act features a member of royalty on the run while a couple of protagonists find the main hero on a desert planet, resembling the original drafts and by extension Star Wars: Episode I. Because of this we've arguably never actually had a "pure" first chapter to the original trilogy, even though Lucas eventually had the film serve this purpose anyway.
TonyPH