The same applies to actors, directors, producers - everyone involved in a production down to the backstage workers (if they want to be hired for another production).
Rampant (for good reason) in DVD Bonus Content and interviews is the fact that almost every person involved talks about what a great production it was, how they all got along great, and how this production is the greatest they have ever worked on. The job doesn't end when the director yells "cut!" - famous people involved in large productions with millions of dollars at stake can't go right home and tell their family and friends what a terrible work environment they are in, and how much they hate their (equally famous) boss and coworkers the way a non-famous person may ramble about theirs.
In short, whenever you see someone going on and on and on about how great the production/actors/everything is, they're selling the show regardless if they really like it or not. No matter how troubled the production really was, it's practically required in their contract, so it's ubiquitous in Hollywood and all related industries. The truth can come out later in a person's Compromising Memoirs (which may even reignite interest in said production!), but when the production is released, you want to get people excited about it to sell tickets/copies, and that's easier if everyone thinks you had fun making the production. Can usually lead follow-ups to employ Biting-the-Hand Humor. It is also not that unusual that more savvy (or powerful) celebrities occasionally find ways to let out their frustrations about certain projects, usually by invoking subtler forms of Damned by Faint Praise.
Like they say: Money talks, and when it talks, it doesn't tell the truth, it says what it's paid to say.
Notable Instances and Aversions of this Trope:
- Hilarious Outtakes never show the director or actors getting actually angry about a scene that isn't working, or visibly frustrated with a fellow actor who keeps flubbing a line, though you know it must happen (at most it happens once a scene is repeatedly flubbed, prompting a snarky\annoyed remark along the lines of "why can't you do this right!"). Even Orson Welles' notably profane commercial outtakes don't disparage the product: only the quality of the writing.
- In-universe example: part of the movie Galaxy Quest was how the actors had to continue to sell the show despite how they actually felt about it.
- Megan Fox notoriously failed to do this and openly compared Michael Bay to Adolf Hitler, single-handedly torpedoing her career almost overnight. As it turned out, it was Transformers producer Steven Spielberg who was personally offended, and not Bay himself. Not really surprising, as Spielberg is Jewish (meaning that he's not likely to brush off Nazi comparisons), and one of the biggest films of his career, Schindler's List, was about a German man who saved several Jews from the Nazi death camps by employing them to work in his factories.
- Likewise, Katherine Heigl got a lot of bad publicity for a Variety interview where she slammed Knocked Up, her break-out movie, as stereotypically sexist. She may have had a point, but even many people who agreed with her viewed it as an ungrateful betrayal, and the string of (arguably more stereotypical, and certainly much more mediocre) romantic comedies she made after it caused the Hollywood Hype Machine to turn against her.
- Channing Tatum enthusiastically promoted G.I. Joe: The Rise of Cobra saying that he took the part because he was a fan of the franchise and felt that the story of intriguing. Years later, he admitted to actually not really caring for the film and that he was only in it because he needed to fulfill his contractual obligation to Paramount and didn't want to risk being sued if he backed out.
- Bill Cosby famously told people not to see Leonard Part 6, saying it was a terrible movie. That and the flop of Ghost Dad probably ensured he'd never have a major film career outside of television.
- When Paul Newman learned that a 1966 re-release of his first film, 1954's The Silver Chalice, was planned so as to cash in on his new fame, he took out full-page newspaper ads apologizing for it.
- Robert Pattinson is quite vocal about how much he loathes Twilight, Stephenie Meyer, and the fans. It's made him hugely popular with Twilight's hatedom. Kristen Stewart has also criticized it.
- Doctor Who:
- In this interview about the TV movie, Paul McGann and Daphne Ashbrook have nothing but nice things to say about almost everything... except Eric Roberts, who was apparently standoffish, rude (making personal remarks about McGann being "effeminate"), and sometimes "amazingly bad".
- Subverted in an interview with Tom Baker, the Fourth Doctor, wherein he one moment praises Paul McGann's performance in the TV movie as "fabulous", which he immediately follows up by making an incredibly casual and off-hand admission in the same breath that he "never saw the film." More likely than not though, this was meant to be humorous, as it is very in-character for Tom Baker to constantly contradict himself in interviews for laughs.
- Steven Moffat was in a notorious video where he and a bunch of other Doctor Who Wilderness Years BNFs got drunk together, in which he said the show was awful and slagged off every actor to have played the Doctor to prove his point that Peter Davison was the only good Doctor. Even though the show was considered dead for good by that point and he was never likely to work on it, even though he was visibly drunk, even though he's been involved in some amazing Who stories since then and even though he's repeatedly apologised and insists he was only doing it to troll Paul Cornell, many fans are unwilling to forgive him for it due in part to the fact that there's little to suggest he wasn't being honest about his feelings, and in part to just how vicious some of his comments were.
- Louise Jameson, who played the Fourth Doctor's companion Leela, insisted to the press that she was quitting the role because she'd felt the character of Leela had been taken as far as she could go, and because she wanted to get back to working in theatre. This was all true but also was to tactfully avoid mentioning the fact that Tom Baker had been bullying her on-set. Almost twenty years later she declined to appear on a Tom Baker interview for This Is Your Life because he'd hurt her deeply enough that she didn't feel comfortable exchanging harmless Witty Banter with him as if nothing had happened. He did eventually apologise to her and the two have since made up and apparently adore each other. In a DWM feature in the November 2014 issue of Doctor Who Magazine she wrote a 'letter to her past self' about it and assured the young Louise that, as strange as it seems, she loves Tom Baker nowadays, now that he's 'mellowed' and is 'nothing but generous'. Aw.
- Janet Fielding, who played the Fourth and Fifth Doctor companion Tegan Jovanka, went on stage at a '90s Doctor Who convention and announced that a show that treated female characters as badly as Doctor Who deserved to have been cancelled. She may have had a point, but remarks like that coming so soon after the cancellation (and trying to talk about feminism to an audience of male nerds) did little to endear it to the fandom at the time.
- There are numerous examples of actors, writers, directors, etc. acknowledging flaws in the stories, and saying other negative things about the process of making them on the documentaries and commentary tracks for the Classic series DVDs. Of course, it helps that the releases were all released anywhere from one to five decades after the story they're talking about was made. Their honesty, if anything, improves the quality of the DVDs.
- Quite averted with The Hobbit: The Battle of the Five Armies. While not bad-mouthing anybody, the behind-the-scenes coverage of the movie's production was incredibly candid about the troubles that occurred. This includes Peter Jackson walking onto set one day and literally not knowing what he was supposed to be shooting as he had run out of script to film, resulting in the production being halted so he could finish the rest of it. It averts the example above as well as the documentaries came out only a few months after the film itself.
- The Titan A.E. DVD Commentary provides at least a partial aversion. Don Bluth and Gary Goldman had nothing but good things to say about each other and the others that worked on the film, but they were not shy about badmouthing 20th Century Fox and their Executive Meddling that starved their movie of time and money (to the point that they were essentially forced to turn out a half-baked product they weren't happy with). By the time their commentary was recorded the movie's disappointing box office numbers would have spelled doom for their careers and Fox Animation Studios as a whole, so they really had nothing to lose.
- Prior to the release of Star Wars: The Last Jedi, Mark Hamill was critical of the handling of Luke Skywalker, at one point telling Director Rian Johnson "I pretty much fundamentally disagree with every choice youve made for this character." When the film was released and Luke's characterization became a massive point of contention among viewers, Hamill's words became a rallying cry for the detractors. Hamill himself has since said that he regrets making his criticism publicly known and that he "got" Johnson's direction after he made the comment.