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In The Empire Strikes Back, Han, Chewie, and Leia are herded into a dining room, which turns out to contain Vader and Boba Fett. Vader says "we would be honored if you would join us," they go in, the doors close, then the film cuts to the next scene. Since the rebels aren't tortured until later, and aren't even questioned when tortured, what exactly happens in that room?

I know it's likely just supposed to be left to your imagination, and Robot Chicken had a great take on the scene, but do any canon (i.e., the old Expanded Universe/"Legends" canon) sources expand on it?

The original 1978 Leigh Brackett draft script for the catchily-named "Untitled Star Wars Sequel" contains a far more extensive 'Dining Salon' scene in which Vader reveals his plans like a great big dummy.

DISSOLVE TO
INT DINING SALON – NIGHT

Everyone is at table, Darth Vader presiding at the head; he is neither eating nor drinking. Lando eats with a good appetite. The others Chewbacca sits quiet, not touching anything. Han and Leia have lost their appetites but both... and especially Han... are drinking wine. Looking at Lando and contemplating murder.

LANDO : You see, Han, I had no choice. I knew who the Princess was the moment you landed, I knew that the Empire wanted you both... and the Empire is important to me. I suppose that I really owe my comfortable existence here to the Empire, though I'm not a part of it, and I continue to trade with both sides. So I really had no choice.

HAN : I hope you were well paid. You know what’s going to happen to us.

LANDO : Lord Vader assured me that nothing would happen to any of you.

LEIA : Ha! I’ve enjoyed Lord Vader’s hospitality before. What’s the matter, Lord Vader? Have you gone so far beyond humanity that you no longer need to eat, or drink?

DARTH VADER : I have had to forego those simple pleasures, like an along with many others; but there are compensations I survive. And you’re forgetting something, Princess. When we entertained you aboard the Death Star, you had information that we needed. Now you have nothing.

LEIA : Then... I don’t understand.

DARTH VADER : Luke Skywalker.

HAN : Luke!?

LEIA : What about him? But I don’t know where he is... or even if he’s still alive.

DARTH VADER : He’s alive. And he’ll find you, because he loves you.

HAN : That’s ridiculous. He could search for...

DARTH VADER : He’ll find her. And until then, we wait. I would advise you against any rash moves. I’ve set a guard on the Falcon so there’s no escape that way. I shan’t lock your doors unless you make it necessary.

LEIA : It’s impossible. Luke hasn’t the faintest idea where we are. You must be out of your mind.

DARTH VADER : Then there’s no need to worry, is there?

LEIA : Any time the Empire is around, I worry.

She rises and strides out, not excusing herself or looking back.

HAN : Guess I’ll go, too. The air’s bad in here. Coming, Chewie Chewbacca?

Chewie growls. They go out. Darth Vader looks after them. Lando glances at him uneasily, then goes on with his meal. But he too seems to have lost his appetite.

Future drafts removed the scene itself but kept the major part of Lando's dialogue with Han/Leia.

The only possible answer I can think of is whether or not they elaborated that scene in the various screenplay drafts. Per the Wookieepedia page for the Empire Strikes Back, in the behind the scenes section, they mention that the screenplay went through multiple drafts by multiple authors. Unfortunately I have never seen, nor do I know how to acquire, previous versions of screenplays that were not used.

Link and caption:

http://starwars.wikia.com/wiki/Star_Wars_Episode_V:_The_Empire_Strikes_Back

Lucas hired screenwriter Leigh Brackett to write a screenplay based on
his story treatment. Brackett finished the first draft on February 23,
1978 but she died soon afterwards. According to Lucas, he didn't like
the direction in which her screenplay went. Without a writer to fix
it, he was forced to write the second draft himself throughout
March[4]. Major plot changes were made compared to the first draft and
the storyline that is in the film was formed in this draft. One of the
most notable changes was making Vader Luke's father[4]. When Lawrence
Kasdan finished writing the screenplay for Raiders of the Lost Ark he
was hired to rewrite and improve Lucas' draft. He wrote a few more
drafts before the script conference was held in November between
George Lucas, Lawrence Kasdan, Irvin Kershner and Gary Kurtz. After
some additional input, the shooting script was finally formed. Minor
changes to dialogues came from Kershner and actors throughout the
filming and were mostly approved by Lucas.

Edit: Apparently it is possible to find at least some of the older drafts of the Empire Strikes Back screenplays. Following link leads to the online pdf version of Leigh Brackett's original draft, which was apparently leaked online sometime around 2010, according to these: source 1, source 2, source 3. Perusing the Brackett draft, around segment 193 INT DINING SALON - NIGHT, it has a dinner table dialoque between Han, Lando, Vader, and Leia.

http://scyfilove.com/wp-content/uploads/2010/05/Star-Wars-The-Empire-Strikes-Back-Brackett-Draft.pdf

Also, at the following link can be found text versions of the fourth draft and public draft, neither of which have any additional dining room interaction following the initial scene. Meaning that it was taken out between Brackett's draft and the fourth draft.

http://starwars.rossiters.com/tesb_screenplays.html

They have a weird Mexican standoff in the ESB radio drama where Han tries to get Chewie to agree to "take" Vader with him, but Vader forces their hand by saying he'll strangle the Wookiee if Han doesn't play ball. It basically extends the film scene, though, doesn't go into the world's most awkward dinner party ever.

What if by now Vader suspects Leia might possibly be close to him. His daughter, Luke’s twin? There is something familiar about her, but he is not sure. He questions Leia about parents, her age and date of birth and her feelings for Luke. He knows he has a son so what if there were twins, but quickly dismisses the idea. But.......just what if? It explains why he prevents bounty hunter from firing at Chewie, afraid it might hit Leia and then his decision to put have Leia and Chewie put in his ship.

It's only ever going to be speculation. It was just never written cos nothing in the scene could be more shocking/tension building than the reveal itself.

Once Vader had the gun it was over, they would simply have to surrender. Vader might well taunt the Princess a as he did every time they came face to face. However he would be more likely to simply say "take them away".