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I think the most important part of role-playing a character is having some sort of goals that the characters wish to fulfill. That's a good starting point as any for how to structure a roleplay-only scenario: what sort of goals do your players' characters wish to fulfill? It may be hard to 'force' this right now, since I'm guessing that the players did not choose a lifelong ambition or similar when rolling up their characters, if it's the first time they are playing tabletop RPGs. But in that case, this is a perfect opportunity to flesh out their characters as they are talking to NPCs and such.
Maybe the Dragonborn wishes to learn more about his heritage, or the revenant wants to know who murdered him, etc. Create plot hooks based around the characters and you will realize how they will want to roleplay much more.
After this, it's a good idea to think up of some interesting characters. To make an NPC interesting, you don't need to write an entire character portfolio describing their life from birth to today. But do think of characters you've liked in books, movies, TV shows, etc., try to distill what you liked about them, and add that in. Think up of some character quirks that you can add, fantasy races that you like, accents that you could do, and general things that will make the NPC more interesting to talk to. Give them names and motivations to make them stand out, too.
Now that you have both the plot hooks and the characters, try to group them up. What plot hooks would fit what NPC? Maybe Balgron the Fat, goblin of high regard, previously hired a Dragonborn mercenary who looked a lot like the Dragonborn PC. These will make your NPCs an integral part of the plot.
Finally, think up of details to add to the dinner party. Will there be a contest? How about a ballroom dance scene? What food will be served? What is the purpose of the feast? having a list of names will also help you add in incidental NPCs. If a player decides to talk to the waiter, you can pull out your list and describe Merric, a halfling with delusions of grandeur and rapier wit, instead of just Bob the waiter.
Hope these tips are helpful!