Eli Roth gets to do some of the more gruesome killings in the film. Coincidence? And this isn't the first time he's played a "bear"—in Death Proof, his character was named "Dov," which means "bear" in Hebrew.
Mike Myers doing a character with a faux-English accent who's involved with international espionage? Yeah, baby.
Enforced Method Acting: Quentin Tarantino decided that he wanted Bridget von Hammersmark's death scene to be very realistic so Tarantino put on a Nazi uniform, took Christoph Waltz's place, and choked Diane Kruger until she was unconscious.
Fake Brit: Michael Fassbender who plays Lt. Archie Hicox, the Quintessential British Gentleman, lives in London but is in fact half-Irish, half-German. Mike Myers (Canadian) plays another quintessential Brit, General Fenech. And finally, Rod Taylor, who plays Prime Minister Winston Churchill, is Australian. Tarantino talked Rod Taylor out of retirement to play Churchill (which turned out to be his last role), even though Taylor suggested Albert Finney (who had played Churchill to great acclaim in The Gathering Storm (2002)).
Fake Nationality: Christian Berkel, who plays a French bartender, is actually German.
Fatal Method Acting: A near miss. During the climactic scene, the fire raging through the cinema was completely real. Unfortunately, it began to get out of hand, and the two actors in the scene were only wearing a jelly to protect their skin; the rest of the crew had fire suits. Ten seconds after Quentin Tarantino called 'cut' and everyone rushed off, the platform the actors had been standing on collapsed. The heat was so intense (2000 degrees fahrenheit) one of them passed out afterwards.
Promoted Fanboy: Mike Myers was reportedly a Tarantino fan, which got him his cameo in Chapter 4 "Operation Kino".
Similarly Named Works: The "Inglourious Basterds" misspelling was likely to avoid a lawsuit by the copyright holders of the 1978 film "The Inglorious Bastards", which was also a World War II film. Though it might also be a reflection on Raine's illiteracy.
Also, at one point Ennio Morricone was onboard to compose the score but backed out due to his belief that he would be unable to work efficiently in the amount of time given.
In the screenplay of Inglourious Basterds, the first part of the film set in Paris was intended to have been filmed in black and white, using entirely natural lighting, in reference to the French New Wave. This was probably cut by Executive Meddling, on the grounds that most of the audience wouldn't get the reference, and came here to see an action movie, get back to the killin' already!
As Quentin has noted, he at one point responded to the length of the material he'd written with a decision to make it a miniseries. As it stands, he's noted that he has enough for a second film, which he may or may not make eventually.