Before we get too deep into this, this article is likely to contain spoilers -- both major and minor. Read at your own peril!

• Some of the characters may have familiar last names -- Vic and Vinny Vox are likely related to Cardinal Sanctus Vox, one of the Legion bosses. The timeframes are inexact, but there are at least a few generations separating them.
• The Clients for general contract targets include The Midnight Tinkerer, who you may also know as Riker Maloney, and Anton Laffrey, who is possibly related to Aisling Laffrey.
• You may have seen some familiar faces around the Rogue's Harbour: Tarkleigh, Hargan and many others can appear at random. In fact, you can also see any member of the Immortal Syndicate except the Mastermind, but only if they aren't currently part of the Immortal Syndicate (ie. you've 'removed' them).
• Gianna and Adiyah are both voiced by the same actress -- Elle Wooton. Any time you hear Gianna and Adiyah talk at the end of a heist, she's just talking to herself!
• Rogue's Markers took many different forms during writing and development. My personal favourite was 'Prints', coins with a fingerprint pressed into them, making them hard to counterfeit and tying it into the criminal theme (don't leave prints behind!). After that, they were 'Seals', and were the wax seals from completed contracts. Apparently, people would hear 'Seal' and think of the animal, hence they are now Markers.
• Storylines underwent a bunch of changes over Heist's development cycle. Ideas that we considered and cut include: Having your first heist be larger, busier, and end in a betrayal that would set up a central rival early on, a story in which Gianna reveals her fate is due to a lecherous and abusive theatre director (I wanted a boss fight that was a pursuit through the backstage area of a theatre), and a sequence in which the Vox twins invade the Rogue Harbour for an early version of their boss fight.
• Hana, Kurai's cat, used to be called Beans, because I thought that was an absolutely adorable name. In the first draft of Kurai's quest, Beans the cat was killed in the aforementioned Rogue Harbour raid. After a few people at GGG read the draft, "For Beans" started appearing on whiteboards around the office. Then cans of beans started appearing around the office, including one on my desk. People got really attached to Beans.
• Karst used to be filled with even more bluster, and was even more cowardly than he currently is.
• We added somewhere in the vicinity of 2800 new dialogue entries. Prior to Heist, all of Path of Exile, including past leagues (even Betrayal!), totalled about 6000 dialogue entries.
• We created a conversation system specifically for Heist, but that we can generalise for future content. For Heist, we added 268 conversations, including one that Vinderi has with himself.
• There were several weeks between the first dialogue recording and the last, with each character recorded individually. It took a lot of work to piece the lines together and tinker with pauses and overlaps to get them to sound natural-ish.
• I had the most fun writing Vinderi. My favourite line out of literally all the dialogue is "Anytibbs, time."
• Writing Kurai's heartbreaking scene was particularly emotional as, during Heist's development, my own cat got sick and had to be put down.
• The Rogue Harbour was originally going to have dynamically moving NPCs and more conversations between them, but then Lockdown 2 happened. For that version of the Rogue Harbour, I wanted Tibbs to be sitting by the fireplace sewing or whittling toys, and Karst, Tullina, Isla and Vinderi playing poker. Karst would keep losing and get angrier and angrier, Tullina would keep goading him on, Isla would use their distracted fighting to cheat, and Vinderi would undo the cheating when Isla wasn't looking. It was elaborate, and probably good it didn't happen. There are still a few dialogues that can trigger around the Rogue Harbour, however.
• Kurai's name is an anagram of Karui, which I only realise once we'd already recorded a lot of her dialogue. It wasn't intentional.
• I tried to make one of the characters an ethical cannibal (again), but that was cut. One day...
• We might try to use a few of these characters again in the future. There are some I'm keen to explore further.

There are probably a lot more little bits and pieces I'm forgetting, since Heist was one of the busiest leagues I've worked on. I've seen a lot of feedback and it really helps knowing people do, in fact, care about lore, stories and dialogue in Path of Exile. And all that feedback is helpful in polishing what we work on next.