Blooper: Killer BOB is a rare character who is born of a blooper. Lynch had been toying with the idea of giving Silva a role on the show already, but accidentally showing up reflected in Sarah Palmer's mirror is what clinched it.
BOB, played by a crew member who accidentally ended up in a pivotal shot. They could have reshot it easily but David Lynch loved the visual of this guy hiding in the shot.
David Lynch himself as Agent Gordon Cole.
Executive Meddling: The studio more or less forced Lynch and Frost to reveal the identity of Laura's killer during season 2, something they'd never intended to do. They were being pressured to reveal this as early as the end of the first season.
Fake American: Michael Ontkean (Sheriff Harry S. Truman) is actually Canadian.
Fake Nationality: The Renault brothers are French-Canadians played by two American actors.
Donna Hayward; played by Lara Flynn Boyle in the series but Moira Kelly in The Movie.
Pierre Tremond was recast, since Austin got to be too old.
Post Script Season: Because of the mentioned Executive Meddling, everything after season one more or less became this. It felt incredibly awkward to have Dale Cooper still hanging around in Twin Peaks, even though he didn't have a reason to stay after the killer had been found. Windom Earle was more of a stand-in for Laura Palmer's killer than a real villain.
Production Posse: Kyle MacLachlan first worked with Lynch in Dune and again in Blue Velvet. Also, Jack Nance, who played the protagonist in Eraserhead (along with minor roles in both Dune and Blue Velvet) plays Pete Martell, who discovers Laura Palmer's corpse.
Will's actor, Warren Frost, is the father of series co-director Mark Frost.
Pierre Tremond's first actor, Austin Jack Lynch, is David Lynch's son.
Screwed by the Network: Twin Peaks was renewed for the second season, but the network moved it to one of the lowest-rated timeslots on television, Saturday nights at ten.
A spectacular case happened in Germany. RTL held the rights for Twin Peaks in 1991, yet competitor Sat1 outright spoiled the whole series by telling the name of Laura Palmer's murderer on their teletext. As a result, the series wasn't such a big hit in Germany.
Short-Lived Big Impact: With only two seasons and 30 episodes, it popularized the Quirky Town genre in American television, having descendants such Picket Fences and Northern Exposure that ran much longer than Twin Peaks itself. Also, the amount of surrealism, eccentric humor, and horror in it were highly exceptional for a mainstream American drama series of its era, but such elements became much more common in television in its wake in the 1990s and 2000s. It also helped to popularize the trend for TV series with cinema-quality visuals, now practically the industry standard in scripted television. And that's not to mention its impact on gaming, most notably as a source of inspiration for the Silent Hill series.
The villain BOB was created/cast when set director Frank Silva's reflection accidentally appeared in a mirror when filming the last shot of the pilot where Laura's mother has a frightening nightmare. Earlier, Silva had trapped himself in Laura's bedroom, endearing him to Lynch, which caused him to shoot footage showing him looking up from the foot of Laura's bed. His serendipitous appearance in the pilot just cemented his place. According to Wikipedia.
Two other things from the pilot: When Cooper first examines Laura's body, a fluorescent light keeps flickering — the light they were using really was malfunctioning, but David Lynch liked the eerie disorienting effect this had, so it got written in as a transformer malfunction. And in the same scene, an extra misheard Cooper's line "Would you leave us?" as "what's your name?" and, thinking Kyle MacLachlan was breaking character, said his real name. The awkward moment that ensued got left in as a momentary aversion of Realistic Diction Is Unrealistic.