Mercer Frey, the leader of the Thieves' Guild, is voiced by Stephen Russell, the voice of Garrett, the infamous protagonist and master thief from the Thief series of games.
General Tullius is a gruff, no-nonsense officer who is practical-minded and does deep down care about his men and Skyrim itself. He shares these traits with one of Michael Hogan's other roles - Saul Tigh in Battlestar Galactica. Unlike Colonel Tigh, however, Tullius doesn't seem to have alcohol problems. He also looks extremely similar to Captain Bailey, another one of Hogan's roles. He's found inside Castle Dour, and Hogan is known for playing dour characters.
Tullius' second-in-command, Legate Rikke, was also a serious female military officer... on a certain space station.
Paarthurnax teaches the player one of the words of fire breath. He's also voiced by Charles Martinet, who's far better known as Mario.
Serana, the female vampire who accompanies you throughout most of the Dawnguard questline, is voiced by Laura Bailey. This isn't the first time she's voiced a vampire...
Miraak is voiced by Peter Jessop, who's best known as the voice of Sovereign. Guess what happens when you touch the pillars without proper magic?
Beam Me Up, Scotty!: The famous quote from the guards actually goes "Then I took an arrow in the knee", not "to the knee" as it's often quoted.
Doing It for the Art: If you no-clip past the borders of Skyrim, you'll find you can walk all the way to Morrowind and Cyrodiil. They're not at all detailed, just height maps with generic textures and a few landmarks (like the White Gold Tower), but they're still there... even though you'll never be able to reach them without cheats or, theoretically, Game Mods.
Much of it is there for those instances when you can see beyond Skyrim from within the game's proper map. There are plenty of places inside Skyrim that do this, too. If you no clip through the walls of a city, you'll see a partially detailed map of the city again to provide the detail you'd see from certain vantage points even though the city interior is a different map. Likewise, when you're within a city, the exterior of Tamriel is all detailed this way, again to provide what you'd be able to see from within city walls. There's a mod called "Open Cities" that moves some of those cities into the Tamriel map. The Dragonborn DLC takes place in Solstheim, the island just north of Vvardenfell, where the events of Morrowind took place.
Hey, It's That Voice!: Skyrim got a much larger voice-acting budget. See the UESP and IMDB articles for full listings of voice-actors and their roles.
Haruko assigns burglary jobs in the Thieves' Guild.
The leader of the Thieves Guild, Mercer Frey, is voiced by none other than Garrett, fittingly enough.
Many of the Thieves Guild-related NPCs speak with Stephen Russell's distinctive "Garrett" voice in one huge Actor Allusion. He also teaches lockpicking, serves as the Winterhold College's resident shifty-goods-dealer, leads the Forsworn, and works at a meadery for a boss that he hates. (The picklock teacher, Cynric, wears a hood that covers his right eye, leaving his pale blue left eye to gleam in the dark as a visual reference.) He's also several male townspeople.
SnapperCarr and Duke or perhaps Craig Boone show up quite a lot, particularly as Sven in Riverwood, Mikael in Whiterun, Onmund in Winterhold, and Erik in Rorikstead. So now Boone has gotten over grieving for his wife and decides to focus on finding a new woman in his life. Although he still competes with Graham at one point.
No Export for You: The Collector's Edition was essentially this for Australia, selling out within hours of being put up for pre-order due to the incredibly small number of copies allocated to the country. As something of a compromise, Australia had its own exclusive Limited Edition, as a free upgrade for EB Games pre-orders.
Wes Johnson voices both the ghost of Lucien Lachance and Emperor Titus Mede II. This is really the only case where Talking to Himself potentially occurs, since you can summon his ghost to assassinate the Emperor in the Dark Brotherhood questline.
Lynda Carter voices the Nord hero Gormlaith Golden-Hilt in Sovngarde and the Daedric Prince Azura.
Craig Sechler plays the Daedric Princes Hircine and Peryite.
Lani Minella is Nocturnal and the Night Mother. Talk about an evil resume - and this is on top of her providing the low, husky voice for female dark elves.
It's kinda surreal to talk to Elisif's servant, Erdi, and then find out that Elisif herself sounds exactly like her.
What Could Have Been: If the file names seen in the editor for their equipment are any indication, the Stormcloaks were originally going to be called the Sons of Skyrim, with their officers being called the Sons of Talos.
It could be that they did call themselves that originally. The term "Stormcloak" started as an epithet that was adopted by the rebellion. History is full of examples of members of a cause being named after that cause's champion as an epithet only to have it be adopted by the people it was intended to insult. On the other hand, Stormcloak NPC's have 'For the Sons of Skyrim!' as a battlecry, are named as such in certain quest log entries, as well as being the name of some of their campsites on the map.
It seems that the Civil War was originally going to have several new features, such as all five main holds being attacked by their opposing side, as well as new radiant quests. At least some of these features were restored with this mod.
According to rumors, there was going to be a Dark Brotherhood sidequest where Erikur commissions you to assassinate Elisif so he can become Jarl of Solitude. Thankfully, it never made it past the cutting board.
An alternate take on that sidequest, "Boethiah's Bidding", was supposed to be a sequel to "Boethiah's Calling" and involved the same task. Some of the code is actually in the game, but the fact that it was never fully implemented makes the quest Unwinnable even if you force-start it with the console.
According to Todd Howard, shortly before working on Skyrim, Bethesda were in talks to make titles based on A Song of Ice and Fire and despite some heavy interest, ultimately turned it down to continue developing the Elder Scrolls and their own lore. The return of the Dragons and Civil War in Skyrim (plots that feature heavily in both works) may also have been partially inspired by this, although this has yet to be confirmed by any official sources.
Miraak, from the Dragonborn DLC, had a bunch of lines cut from the final game. These lines make him more of an Anti-Villain (such as regretting having to kill you), and even hint that he was originally slated to live at the end of the main quest.
Mephala's Daedric Quest has some cut content that implies that Jarl Balgruuf would have been killed by his own children, with his brother taking over as Jarl. Remnants of this are still hinted at within the quest and the plot of Balgruuf's son having been corrupted by Mephala is never resolved.
Considering the dropped Balgruuf and Elisif murders in game, Skyrim, as originally written would have been much more Pro-Stormcloak, ultimately leaving the Empire with only one Reasonable Authority Figure Jarl, who's probably going to die of old age soon, with the remainder being scheming backstabbers. While Elisif's was dropped early in development, Balgruuf's made it all the way to early beta testing. It was ultimately the Beta Testers who pointed out that killing off reasonable authority figures on the Imperial side, especially the one who trusts the Dragonborn from the start of the game, made players choosing to go Imperial a Shoot The Shaggy Dog Story and would ruin the moral ambiguity of the Civil War storyline that's loved today.
Word of Saint Paul: The idea that the Thalmor want to end Talos worship as part of a plot to destroy the mortal world and Ascend to a Higher Plane of Existence doesn't actually appear in-game, but comes from forum posts by Michael Kirkbride. While responsible for a lot of Elder Scrolls lore (especially Morrowind) and the creation of the Thalmor originally, he was not directly involved in writing the plot of Skyrim beyond the initial idea stages (that happened during Morrowind and early Oblivion).