Enforced Method Acting: The actors hung out before shooting began so they'd become close, and the diner scenes were held till the end of the shoot so they'd have bonded by then. Also, when there were conflicts between some of the actors, Levinson made all of them stay in what was known as the camaraderie camper.
Executive Meddling: As mentioned above, the studio didn't like the movie, and was going to dump it, until critics such as Pauline Kael raved about it.
Stuart Markowitz is Bagel, an aluminum siding salesman trying to get Boogie to work for him.
Science Marches On: At one point, Fenwick is watching a quiz show on TV, and one of the answers involves knowing the fact one half of Mercury faced the sun while the other half didn't. Astronomers honestly believed it at the time the movie was set, but the following decade, this was discovered to be false.
Throw It In: Steve Guttenberg couldn't react coherently to the answer of "Presley", but Levinson kept it in because it was funnier.
Generally speaking, most of the dialogue at the diner was improvised, especially Modell's, since Paul Reiser was so good at it.
What Could Have Been: Part of the script involved Eddie and some of the others going to see the 1959 NFL championship between the Colts and the New York Giants, but Levinson couldn't afford to shoot anything at the football stadium, so the sequence had to be scrapped.
As noted above, Levinson wrote, produced and directed a pilot for a tv version of series, which would have featured Paul Reiser as Modell.
Andrew "Dice" Clay tested for the role of Eddie Simmons.
Write What You Know: The film is based on Levinson's experiences growing up in Baltimore (the football quiz, for example, was something he claimed a cousin of his did). There has been speculation about which of the characters was based on Levinson, but Levinson has always insisted that no one character was based on him; all of them were partly based on him.