The Aryan-esque Imperial officer who uses Leia as a body shield during the Bespin firefight with Luke is actor Jeremy Bulloch, who spends the rest of his scenes in the movie playing Boba Fett in face-concealing armor. The choice to use him as the officer was less about letting him show his face on-screen than that he fit the blond, blue-eyed Nazi type they thought was appropriate and was already on set.
In the Special Edition, the voice-over line "The first transport is away" during the Rebel evacuation was re-recorded by Mark Hamill.
George Lucas hated the process of scripting the first film, so he hired noted pulp SF novel author and Golden Age Hollywood screenwriter Leigh Brackett to write the script for Empire. She wrote one draft, but died of cancer soon afterwards. As a result, Lucas wrote the next few drafts himself, before asking Lawrence Kasdan to do revisions. Incidentally, the famous Luke, I Am Your Father reveal wasn't in Brackett's draft; Lucas only added it when he started rewriting the script himself. Although most of her version was rewritten, two contributions by Brackett did end up making it into to the final film: the planet names 'Hoth' and 'Bespin'.
Yoda almost suffered this in the prequels and sequel because George Lucas originally asked Jim Henson to play him.
John Barry, who had been production designer on the first film, was earmarked as a possible future director for the series by Lucas, and given the second unit director's job on this film to give him some experience (he had previously signed on to direct Saturn 3, but it fell through when it turned out Barry had no real idea how a film shoot actually works). Sadly, he suddenly fell ill with meningitis halfway through production and died just a few hours later, forcing the team to find a new second unit director and robbing Lucas of the obvious choice to direct the next film.
David Prowse:"This for me was my favourite of the Star Wars movies. We had a wonderful director, Irvin Kershner, who I rank as one of the best directors I have ever worked with. It was, of course, a big reunion for us all, and by now we all knew that we were involved in a cinematic phenomena. [...] The old team were back together, plus new cast members like Caroline Blakiston who played Mon Mothma, and my old friend Bruce Boa who played General Reikeen. Bruce and I remained good friends right up until his death. [...] and I thought the carbon freezer scenes were sensational [...]"
Beam Me Up, Scotty!: Vader says "No, I am your father", not "Luke, I am your father." He also doesn't say "Impressive. Most impressive. But you are not a Jedi yet!" These are two separate lines.
George Lucas called Empire "the worst Star Wars film". Yes, really. Though, then again, problems with filming and its original reception need to be remembered, since at the time of its release the film was considered worse than the original by critics and many moviegoers.
David Prowse, while pleased with the film overall, was furious at Lucas and Kershner for keeping him in the dark about the big reveal because he would have acted out the scene completely different had he known the real twist.
Enforced Method Acting: When shooting the big scene between Luke and Vader, David Prowse said "No, Obi-Wan killed your father", and that's what the entire crew of the film thought would be said when James Earl Jones dubbed in his lines. Only 5 people (Lucas, Jones, Mark Hamill so he would react correctly, writer Lawrence Kasdan, and director Irvin Kershner) knew the actual line.
And Hamill didn't know until they were on set and ready to film. Kershner took him aside and told him moments before. They were very determined to keep the big reveal a secret.
According to a 2016 interview with Mark Hamill, the extreme precautions turned out to be totally justified; not long after the scene was filmed, newspapers were reporting the "big twist" of Obi-Wan killing Luke's father - meaning someone on the set that day went and blabbed.
David Prowse was furious about learning that he'd been kept in the dark, saying that his body language would have been completely different had he known what the real line was supposed to be.
Executive Meddling: Defied. George Lucas wanted an opening scroll for Empire Strikes Back to match the one he did for Star Wars. The Director's Guild of America attempted to force Lucas to have opening credits instead.note During this time, it was required to have the credits in the opening to give proper due to the director of the film. The Guild gave Lucas a free pass with the first film because he was the director himself, but since Lucas had someone else direct Empire, they were no longer letting him off the hook. Long story short, Lucas decided to quit the guild and form his own studio so he'd never have to deal with the meddling from guilds again.
It Will Never Catch On: Despite the surprising and massive success of Star Wars, there were serious doubts that the sequel would be anywhere near as good. Irvin Kershner nearly turned down the director's chair because he felt he could never make it as anything other than "the second one." This made sense because doing sequels was considered drudge work by most in Hollywood (hence why Marlon Brando refused to reprise Vito Corleone in The Godfather sequels) at the time. His agent convinced him otherwise, and now Empire is almost universally acclaimed as the best film in the saga.
Lying Creator: To make absolutely sure the big twist wouldn't leak, only Lucas, Irvin Kershner, James Earl Jones, and Mark Hamill knew about the real line, with the script featuring Vader saying "Obi-Wan killed your father." The rest of the cast and crew only learned the truth at the movie's premiere (when the true line was spoken, it allegedly prompted a Big "WHAT?!" from Harrison Ford), and David Prowse was quite upset with Lucas afterwards, saying his physical acting would have been completely different if he'd known the real line.
James Earl Jones would also state in a later interview that this was his reaction when he saw Vader's lines in the script; he was sure Vader had to be lying about it, and only with some difficulty were Lucas and Kershner able to persuade him otherwise.
A theatrical trailer narrated by Harrison Ford features C-3PO ripping a decal off a door. This is from a Deleted Scene where 3PO tricks some Stormtroopers into walking into a Wampa containment room.
There was also a shot where Vader leans forward for some reason (supposedly after his fight with Luke, as if to throw him off himself). Some speculate that this is from him trying to use the Force to pull Luke back up to him, a scene described in the children's picture book of the movie.
Not to mention a completely different shot of Luke and Leia making out. Notably, they appear to be alone in this scene, as opposed to the one in the film proper which was in front of the regular gang. Maybe the filmmakers were just shipteasing, or (less likely, considering how we know the writing process went) they were trying to throw off the scent of the twist in the next film.
No Stunt Double: With the exception of being sucked out of a Cloud City window and the use of some puppets, Mark Hamill did all of his own stunts.
Off-the-Shelf FX: The shot of Luke force-jumping back out of the carbon freeze chamber was actually a puppet made from a 12" Kenner toy and some masking tape.
In the original release of The Empire Strikes Back, Palpatine was portrayed by Marjorie Eaton (sort of: Her appearance was superimposed with that of a Chimpanzee's eyes, and her voice, similar to that of Darth Vader's actor, would be dubbed over by Clive Revill). The DVD and Blu-Ray releases of The Empire Strikes Back would replace her with Ian McDiarmid.
Likewise, Boba Fett's voice was changed from Jason Wingreen's voice to Temuera Morrison's voice in the Original Trilogy's first DVD release to reflect Boba's status as a clone of his father, Jango Fett.
Contrary to popular belief, the scene where Luke got bashed around by the wampa was NOT due to Mark Hamill's car accident.
Scully Box: Carrie Fisher stood on a box for many of her scenes with Harrison Ford in order to make up for the height difference and have her appear in the frame with him. She was a foot shorter than him.
When they just couldn't get Han's response to Leia in their last scene right, Irvin finally just told Harrison to get in character and they would just run the scene without him being given a line to see how he would react, and he just blurted out "I know." The original line was "I love you, too." Ford argued that Han Solo would never say such a thing directly, much less repeat someone. Lucas and Kershner agreed, though Carrie Fisher was pissed at Harrison for changing the line without running it by her first.
In the original script, when Lando is about to lead Han, Leia, and Chewie into the trap set by Darth Vader, Lando offers his arm to Leia, as a gesture to lead her down the hallway and she accepts it. Ford ad-libbed Han coming up behind Leia and offering his arm to her at the exact same moment, to imply that Han was jealous.
Troubled Production: While not quite as brutal as the production of the previous film, Empire nonetheless had a fair share of behind-the-scenes turmoil.
Production went over budget (triple that of the original in fact) and behind schedule, and the Hoth location shoot in Norway was plagued by a strong snowstorm (How bad? The scene where Luke escapes from the Wampa lair was achieved by opening the door to their hotel and filming Mark Hamill running outside.). The film also suffered overcharges on location shooting by locals eager to cash in, given the success of the predecessor; Lucas would shoot Return of the Jedi under a fake name to prevent this price gouging again.
New director Irvin Kershner spent a lot more time setting up takes than Lucas did, causing a bit of friction between the two. Kershner and the actors also changed bits of dialogue from the shooting script, not all of which were approved by Lucas.
Lucas wanted to keep the film out of any studios hands and financed it himself, but he was forced to take out a loan with 20th Century Fox as his security, on the condition that Fox would receive a greater percentage of the film's profit.
The Yoda puppet was made of a less-than-optimal material, resulting in it being quite a bit heavier than what Frank Oz was used to from his time with the Muppets. The strain put on his arms meant the scenes had to be shot on a quite erratic schedule.
In Leigh Brackett's initial script draft, Vader is explicitly not Luke's father; Anakin appears to Luke on Dagobah as a Force Ghost, alongside Obi-Wan. As well, it's revealed that Luke has a twin sister: not Leia, but someone else called Nellith. She's mentioned but never seen, in what was intended as a Sequel Hook for later episodes (note the plural). This was probably what Lucas had in mind when he inserted the "there is another" line. Then Brackett died, and the filming on Empire was so chaotic that Lucas decided to wrap up the saga with only one more film, which led him to make the "other" into somebody we already knew and turn Vader into Anakin (while also developing backstory that would later inspire the prequels).
Yoda was originally named Minch, who dueled Obi-Wan's ghost, and Lando was a clone.
An earlier draft had Luke's reason for not leaving with Lando and Chewie at the end being that his Jedi training was more important. Believing that this would make Luke seem less sympathetic, Irvin Kershner had it changed to where Luke was still recovering from his injuries and that rescuing Han would be his first priority once he was fully recovered.
One of the first ideas for Lando Calrissian was to have him as a clone who survived the Clone Wars who leads legions of clones on a planet they settled on. Another idea had Lando as the descendant of survivors of the Clone Wars, born into a family who reproduced solely by cloning. Originally, his name was "Lando Kadar".