Enforced Method Acting: In the hospital scene, James Caan decided to stare coldly at J. Jay Saunders, who played the doctor. This really frightened Saunders, and his reaction in the scene is genuine.
Retroactive Recognition: Dennis Farina was a Chicago PD detective at the time who served as a consultant on the film and made his acting debut as one of Leo's silent henchmen. Farina became a familiar and busy actor for the rest of his life, following this film with roles in Mann's Manhunter and Crime Story.
Throw It In!: In the scene in Leo's house, James Caan improvised mispronouncing "elected" as "elected-ed" to show that Frank was an uneducated man who tried to project an air of sophistication.
Alternative Foreign Theme Song: Thief II: The Metal Age provides a peculiar example in its German (and only German) localization. The credits video for the game and a few ambients in certain in-game locations use the song "Accingite Vos", by the German band Subway to Sally. Weirdly, all other versions of the game don't include it, to the point that even in the English language version, there isno background music in the credits video. The length of the credits video and how well it syncs with the song seems to indicate it was originally planned to be released as part of the non-German versions too (including the original English language one), but some last-minute copyright issue probably prevented that. Some surviving online news releases from the time of the game's development indicate this might have been the case, as there was no mention of the band contributing music only to the German language version of the game.
The Danza: Keeper Nate in The Metal Age is a nod to his voice actor, Nate Wells (who also voiced Keeper Artemus, creating something of a Continuity Snarl).
Descended Creator: Many of characters in the series were voiced by the developers themselves, something that was a tradition in several other Looking Glass Studios games or games LGS employees had worked on.
Terri Brosius voiced Viktoria in both The Dark Project and The Metal Age, and Edwina Moira, Lauryl and Marla Madison in Deadly Shadows. She also provided some of the voice samples for ghostly figures from the first two games.
Dan Thron voices a whole range of characters, including the "Smart Guard", crime boss Ramirez, Garrett's former associate Cutty, various City Watch members and civilian characters, and even The Eye. He also voiced the first iteration of Keeper Orland during his early bird cameo in The Metal Age.
Nate Wells is the voice of Garrett's mentor, Keeper Artemus, in The Dark Project and Deadly Shadows, and also voices the bit character Keeper Nate in The Metal Age.
The Masked Servants of the Mechanists from The Metal Age are voice-acted by Laura Baldwin, the main storywriter for the first two games. Her voice is quite distorted and modified, though still recognisable.
Randy Smith voice acted Brother Murus in The Dark Project.
Eric Brosius, sound designer and composer for the series, voices the animated statue monsters and some of the Kurshok in Deadly Shadows.
Sara Verilli voices the Library Ghost in one mission of The Metal Age.
Ian Vogel voices some of the mage NPCs in The Dark Project.
Josh Randall voices the Keeper instructor NPCs in the tutorial mission of The Dark Project.
Mike Chrzanowski voices some of the armed thief NPCs in the first two games.
Another dummied out feature was the multiplayer, planned for already the first game, but only implemented into the second one. And even then, the game shipped without the multiplayer feature being playable. After years of modding attempts, the multiplayer feature was restored and polished enough to be decently playable. While Thief multiplayer matches aren't a very frequent occurence, they are played by some of the fans from time to time.
Garrett would have originally retained a sword in Deadly Shadows, but after the decision was made to switch to a dagger, the only remnant of this is a mostly untextured sword model.
A collection of several other features, concepts and NPCs that didn't make it into the final games can be seen here.
Fan Community Nicknames: Thief fans are simply "Taffers", based on the fictional cussword used for the setting's thieves and general low-lifes.
Game Mod: In addition to fan missions for the three original games themselves, there are several mods, total conversions and even a whole Fan Remake that utilise the same basic style of presentation and gameplay.
The Dark Mod, a freeware Fan Remakeof the style of gameplay and esthetics as seen in the entire original trilogy. Runs on the Doom 3 engine and comes with its own separate editor toolkit, Dark Radiant. Fan-developed missions and scenarios can be downloaded directly within the game, via a built-in utility. After spending years (since 2004) in continuous development and testing (during which players needed copies of Doom 3 to run it), it was finally released for free, as a complete standalone, in October 2013. It won PC Gamer's 2013 "Mod of the Year" Award. The game is still in continuous development, adding improved gameplay, technical aspects and new assets through regular updates every few months. The game installer doubles as an updater for just this purpose. In spring 2017, the game successfully got greenlit at Steam Greenlight.
Thievery, a multiplayer mod for Unreal Tournament, with opposing teams of city guards and professional thieves in the vein of those from the series.
Nightblade was a multiplayer mod/game project similar to Thievery, but with more up to date tech behind it. Unfortunately, its development seems to have stalled in recent years, as there's not much news coming out about it.
Lawyer-Friendly Cameo: In the aforementioned The Dark Mod, this applies to names and details of concepts from the original series, due to the game being a freeware fan remake. While the names of the gadgets and equipment don't have a problem with this, the names of factions, creatures and some of the locales had to be modified a bit in order to avoid copyright issues/lawsuits. The changes go like this :
Missing Scene: For some reason, PC versions of Thief II circulated after its original release (for example, a budget-priced reissue several years later) omit all the cutscenes, specifically those in which Garrett is briefed about the next mission. As such, the player often starts a mission with only basic instructions to follow and no context.
Caused by the ancient version of Bink video bundled with the game being incompatible with modern versions of Windows. The original's a compatibility nightmare anyway, being targeted to one of the original iterations of Direct X; websites with patched versions of the rendering libraries will normally have links to updated video libraries, too.
The Other Darrin: Aside of the complete recasting for the 2014 reboot, some of the voice actors for characters from the original trilogy were recast between installments.
Keeper Orland is voiced by Dan Thron in The Metal Age and by Ken Webster in Deadly Shadows.
Interpreter Caduca was voiced by Esra Gaffin in The Metal Age and Paula Rester in Deadly Shadows.
Translator Gamall was voiced by Nancy Taylor in The Metal Age and Terri Brosius in Deadly Shadows. (Both only did her pre-reveal voice, a different actress portrayed her monstrous voice.)
Larkspur is voiced by two different actors in The Metal Age and Deadly Shadows. His appearances in the latter are non-scripted only. In the former, he has a fairly deep voice and only appears in a single cutscene.
Since the early history of Irrational Games is linked with Looking Glass Studios, the first episode of IG's podcast Irrational Behaviour was dedicated to the role some of the LGS/IG developers played in the making of the earlier System Shock game and the making of the Thief series. For the particular bit of the episode that discusses Thief, go here.
You'd never know it from just playing, but Garrett, Karras, Raoul the beggar, certain Hammerites and "Benny" the comedic guard are all voiced by Stephen Russell. This happens a lot for many other characters, too.
Done in a slightly more in-universe sense for the Children of Karras, the robots whose voices are simply Karras' voice recorded.
The infamous Bow Upgrade that was supposedly hidden somewhere in the first game. This is an old Running Gag that became Hilarious in Hindsight when newly released screenshots and footage of Thief IV came out, showing a more steampunkish compound bow, rather than the simpler one that Garrett used in the previous games. As many fans have noted, Garrett seems to have finally found that long sought-after upgrade !
Some players reported that, in Deadly Shadows a disturbing hint about little Gamall's true nature occassionally appears when you sneak up on her NPC in the Keeper Library. Ultimately, this rumour proved true, as seen in this video. On a less disturbing note, it's an NPC height glitch (as Gamall is the only child-height NPC in the game), not something the developers intended on purpose. Either way, a rather spooky sight...
An archived version of the website with a development log and news about the first game can be read here. An archived version of the first game's original promotional website is here.
Thief II Gold, planned before Looking Glass Studios went under. A journal by a designer hints towards a mission going up against necromancers, and another planned during an active raid. There have been repeated attempts at a Fan Remake of sorts, utilizing what has been preserved or documented about these unfinished missions to create a string or campaign made up of them, as an ersatz replacement for the unreleased expansion pack or Gold edition. So far, only one such recreation of an unmade mission has been finished and made available for download. The remake project has currently entered yet another hiatus.
Concept art for the first three games show several weapon, tool and creature ideas that didn't make it into the final releases. One of the more novel ideas for Deadly Shadows was a mine-shaped "noise supressor"◊ gadget, which would presumably create a small area of total silence around its perimeter, allowing Garrett to be noisier than usual during a heist.