- Acting in the Dark: Anne Hathaway claims she doesn't know if Lureen is lying when she tells Ennis how Jack died. There were two takes filmed with differing motivations, and parts from both of them were edited together in the final film.
- Actor-Inspired Element: When Ennis finds his and Jack's shirts hanging in their childhood closet, Ennis's is tucked inside Jack's. At the end when Ennis takes them home, they are reversed. This was Heath Ledger's idea. Years later, when Annie Proulx was writing the libretto for the opera adaptation of her story, she included the stage direction of Ennis reversing the shirts before hanging them up.
- Author Existence Failure: Editor Geraldine Peroni (best known for working with Robert Altman) passed away before she could finish her work on this film. It was completed by her former assistant editor Dylan Tichenor (later of There Will Be Blood fame). Both were then nominated for the BAFTA for Best Editing.
- Award Category Fraud: The film gives a lot of screentime to both Heath Ledger's and Jake Gyllenhaal's characters, as the film covers their romance and how their lives drift off in parallel fashion during their years apart. As such, one could argue that Ennis and Jack are equally important to the story as protagonists. However, Ledger was put in the Leading Actor category, and Gyllenhaal in the Supporting Actor category (despite him actually having more lines of dialogue), all through the awards season, because Ledger's character has just a bit more emphasis and the two actors didn't have to compete against each other.
- Banned in China: Literally. Despite celebrating Ang Lee's Oscar-winning movie as a triumph for the Chinese, the movie is still banned in the mainland (not the case for Taiwan) for its portrayal of homosexuals. It also got banned in most of the Middle East except for Lebanon and Turkey.
- Beam Me Up, Scotty!: The film's Signature Line is "I wish I knew how to quit you", not "I wish I could quit you" or "I can't quit you."
- Big Name Fan: One of Daniel Day-Lewis' favourite films. After Heath Ledger's death, he dedicated his SAG Award for There Will Be Blood to Heath's memory.
- Cast the Runner-Up: Anne Hathaway was sent the script with the role of Alma in mind. But when she read it, she decided she'd rather audition for Lureen instead.
- Creator Backlash: Annie Proulx has expressed regret over the story, if only for fans who send "ghastly manuscripts and pornish rewrites of the story to me, expecting me to reply with praise and applause for 'fixing' the story..."
- Creator-Preferred Adaptation: Annie Proulx approved of the film, saying that Heath Ledger was exactly as she'd pictured Ennis.
- Deleted Scene: They filmed a sequence where Jack and Ennis help a group of hippies get their car out of a river. Word of God was that they wanted to show Jack and Ennis in a heroic situation. The sequence was disliked by nearly everyone—as it doesn't appear in the original story or the screenplay—and got cut from the final film.
- Enforced Method Acting: In order to get into character and really show Alma's anguish over Ennis' infidelity with Jack, Michelle Williams asked Heath Ledger, who she was dating at the time, and Jake Gyllenhaal to make out behind the camera so her emotional anguish would be realistic. When they weren't passionate enough, she yelled at them.
- Fake American: Australian Heath Ledger as Ennis.
- Name's the Same: Brooklynn Proulx, who plays Jenny at age four, is not actually related to the author Annie Proulx.
- Nasal Trauma: While filming the reunion, Jake's nose was nearly broken from Heath kissing him so hard.
- Never Work with Children or Animals: Ang Lee had already sworn off ever making another movie featuring sheep after horrible experiences making Sense and Sensibility. Apparently, they were even worse here.
- Playing Against Type: Anne Hathaway as Lureen. This was one of several roles that moved her away from the Contractual Purity reputation she had at the time. Anne's agent even decided not to mention her work in The Princess Diaries or Ella Enchanted, and introduced her to Ang Lee as a New York City Broadway actress. He hadn't seen any of her work beforehand and cast her based on her audition.
- Romance on the Set: Heath Ledger and Michelle Williams fell in love on the set, and their daughter Matilda was born just before the film premiered.
- Saved from Development Hell: Screenwriters Diana Ossana and Larry McMurtry bought the screen adaptation rights just days after the short story was published in October 1997 (Annie Proulx strongly doubted that the story would work as a film, but she agreed to let them attempt it), and they completed the screenplay within a few months. After passing from one production team to the next (and being known as "the greatest un-produceable screenplay" going around) for six years, filming finally began in spring 2004, and the film was released nearly two years later.
- What Could Have Been:
- Mark Wahlberg and Joaquin Phoenix were considered for the leads in the film. Although the former was interested, he found the script too sexually graphic, and turned it down. Joseph Gordon-Levitt was also heavily considered for the role of Ennis.
- When Gus Van Sant was going to direct, he offered Matt Damon the role of Ennis. He turned it down, saying:
- Other actors considered for the leads were Ben Affleck, Billy Crudup, Colin Farrell and Josh Hartnett.
- Joseph Fiennes loved the project so much that he met with three different directors, attached at various times, about starring in the film.
- Joel Schumacher was attached to direct at one stage.
Trivia / Brokeback Mountain