Complicated further by the books being an adaptation of only the first two radio serials, while the other three books are original. The final three radio serials, meanwhile, are adapted from the later books.
Also the second radio serial incorporated ideas (such as the importance of towels) that first appeared in the first book.
Beam Me Up, Scotty!: 42 is not "the Meaning of Life". Say it with me, people: It's "the Answer to the Ultimate Question of Life, the Universe and Everything". It doesn't help that Adams himself wrote a book entitled The Meaning of Liff (defining a number of words that didn't exist in the English language, but should have).
Defictionalization: Many bartenders have had a go at reproducing the Pan Galactic Gargle Blaster; not surprisingly, most of the results are almost, but not quite, entirely unlike "having your brains smashed out by a slice of lemon, wrapped 'round a large gold brick". When asked about such drinks on Slashdot, Adams said "Unfortunately, there are a number of environmental and weapons treaties and laws of physics which prevent one being mixed on Earth. Sorry."
Inspiration for the Work: Douglas Adams claimed that the title came from a 1971 incident while he was hitchhiking around Europe as a young man with a copy of the Hitch-hiker's Guide to Europe book: while lying drunk in a field near Innsbruck with a copy of the book and looking up at the stars, he thought it would be a good idea for someone to write a hitchhiker's guide to the galaxy as well. However, he later claimed that he had forgotten the incident itself, and only knew of it because he'd told the story of it so many times. His friends are quoted as saying that Adams mentioned the idea of "hitch-hiking around the galaxy" to them while on holiday in Greece in 1973.
Life Imitates Art: Smartphones and tablets with access to Wikipedia mimic the capability and functions of the Guide with uncanny accuracy, right down to the Guide's questionable accuracy. This might have been a deliberate homage on the part of smartphone and tablet makers as the communicators in Star Trek: The Original Series influenced the design of flip phones.
Life, the Universe and Everything was based on an unused Doctor Who script, Doctor Who and the Krikketmen.note And you can tell: Trillian very much becomes an Expy of the Doctor, with the other characters behaving rather like the Doctor's companions, although this is in keeping with Trillian's intelligence and generally kind-hearted nature. It might have been a second Hitchhiker TV series.
The later radio series were essentially adaptations of Adams' last three Hitchhiker novels, retconning pretty much the entire events of the second series to being merely the delusional rantings of Zaphod Beeblebrox, instead of following directly from them.
Revival by Commercialization: Louis Armstrong's "What a Wonderful World," played at the conclusion of the first phase of the radio series, became popular again because of its use here. It was done again at the end of the TV series.
Science Marches On: The first book has a joke about a planet where, to prevent erosion, the difference between what a tourist eats and what he excretes is surgically removed from his body right before he leaves, so if you go to the bathroom there, it is vitally important you get a receipt. Science has since worked out that most of the matter that a body expels after digestion comes out as sweat or exhaled carbon dioxide, so that wouldn't actually help any.
Self-Adaptation: Douglas Adams' level of involvement with each adaptation of The Hitchhiker's Guide to the Galaxy varies, but the novels and computer game are the ones he had the biggest (or, in the case of the novel, only) hand in, and he delighted in completely reworking the story each time he tackled it. The TV version also had his input, and his last draft of the film before he died was used as the final one with minimal editing.
Technology Marches On: Digital watches sure are a neat idea, huh?note One should remember that when the radio series and first novels were written, digital watches had power-hungry LED displays that needed to be activated by the opposite hand for the wearer to tell time, and the displays were unreadable in direct sunlight. (In the later radio series, and the movie, they were replaced by mobile phones.)
Also, the quip about needing "several inconveniently large buildings" to carry around a copy of the Encyclopedia Galactica.
Unfortunate Name: Slartibartfast. Adams wanted him to have an immense sadness, so gave him a terrible name. He started with FartyFuckBalls, and mutated it until it sounded rude without actually being rude.
Author Existence Failure: Trope Namer, almost; the third book mentions a "total existence failure". Later, of course, succumbed to the trope when Adams died while working on the sixth book; his last published collection of pieces, The Salmon of Doubt, contains an early draft of a Dirk Gently novel that Adams was hoping to rework into a Hitchhiker book.
Creator Breakdown: Regarding the Downer Ending of Mostly Harmless and the mixed-to-negative reaction from fans, Adams conceded, "I just had a thoroughly miserable year, and I was trying to write a book against that background." He intended a sixth book to give the series a better conclusion, but succumbed to Author Existence Failure first.
Executive Meddling: In Life, the Universe and Everything, the Silver Bail of Peace is the Rory Award for "Most Gratuitous Use of the Word 'Fuck' in a Serious Screenplay". US censors were offended, and Adams, bowing to their wishes, promptly changed the offending word to "Belgium". Fortunately, the resulting passage was much funnier, as Adams modified the surrounding conversation to fit the change, as well.
Arthur remarks that he's already met Zaphod, but here it is not so improbable to the audience as the sequence of events are presented in chronological order.
Deep Thought has spent time not only attempting to answer the question, but has also been watching cartoons.
Deep Thought is shaped more like a computer monitor, with a structure resembling an arm to support its chin at the base of the monitor. In the book, Deep Thought is more of a straightforward super computer.
Viltvodle VI is described as a culture of small blue beings in the book, but here it is visited by the crew and inhabited by multiple alien life forms, including an entire church inhabited by humanoids who follow the religion of the Great Green Arkleseizure. The crew visits their religious leader, Humma Kavula, who demands they find the Point-of-View gun.
Frankie and Benjy the mice are the Deep Thought programmers Lunkwill and Fook in disguise, and are killed by Arthur. In the book, they are not related, have no ulterior motive, and do not attempt to lobotomize Arthur.
Gag Halfrunt is quoted in the book. Here, he is shown on a monitor, acting a little different.
Zaphod is the Galactic President in the book, but there is no Vice President Questular Rontok in the novel.
The Point-of-View gun and toaster knife are exclusive to the movie. The scene with the point of view gun being used on Arthur, Zaphod and Trillian is not in the book.
The restauarant at the end of the universe is at the end of the universe in terms of distance, not time.
Author Existence Failure: Douglas Adams died two years before production on the film began, though after he'd finished his parts of the script.
God-Created Canon Foreigner: Many of the differences from previous versions were actually Adams creations, from his own pre-death versions of the script, including the POV Ray, the flyswatters, Humma Kavula, and the romantic elements.
Posthumous Credit: Despite dying two years before production officially began on the film, Douglas Adams is credited as the film's executive producer. He is also given co-credit for the screenplay, but given the film was based upon his novels and had been in Development Hell, this is understandable.
The movie was first optioned in 1982 by producers Ivan Reitman, Joe Medjuck and Michael C. Gross. Adams wrote three drafts for them per his contract. During this time, Medjuck and Gross were considering Bill Murray or Dan Aykroyd to play Ford Prefect, but then Aykroyd sent them his idea for Ghostbusters (1984) and they did that movie instead.
Around 1990, a then-unknown Tim Roth was seriously considered to play Arthur Dent.
The original scripted ending had Zaphod firing the POV gun at Questular.
Questular: What am I so pissed off about? I'm great! I'm zarking incredible! Everyone loves me! WHOOO HOOO!!!
Between the Secondary and Tertiary Phases, Peter Jones (the Guide), Richard Vernon (Slartibartfast) and David Tate (Eddie) all passed away, leaving William Franklyn, Roger Gregg, and Richard Griffiths taking over their respective roles. For the Hexagonal Phase, the deceased William Franklyn was replaced with Douglas Adams' friend and co-author, John Lloyd.
Bill Wallis, who played Prostetnic Vogon Jeltz and Mr. Prosser in the original two series, was replaced with Toby Longworth for the third, fourth, and sixth series due to being unavailable.
Real Song Theme Tune: The theme for the radio show was a snatch of the Eagles' "Journey of the Sorceror", an instrumental from their 1975 album One of These Nights. It would be re-orchestrated for the Radio LPs, and that arrangement was later used for the TV series.
Sequel Gap: There was a 24-year gap between the end of the Secondary Phase in 1980 and the beginning of the Tertiary Phase in 2004. After a brief flurry of activity, there was then a further gap of 13 years between the Quintessential Phase (2005) and the Hexagonal Phase (2018).
Too Soon: The BBC provided a content warning when the episode involving the air attack on the Guide offices (a giant H-shaped skyscraper) was aired shortly after 9/11 — to their credit they didn't postpone the broadcast altogether.
Trolling Creator: Slartibartfast is not named for the majority of the episode he first appears in. Word of God explained that it was a joke at the expense of the woman who had to type the scripts — that after she'd typed this name a dozen times he simply says, "My name is not important".
Michael Palin was the original choice to voice the Guide, but he turned it down.
The seventh episode was originally considered to be a stand-alone Christmas special (owing to both episode six tying up many plot threads and the broadcast date, 24 December 1978) in which Marvin would have been both figuratively and literally the star (of Bethlehem), and by participating in a nativity scene would be cured of his depression. In the end, it was a normal episode devoted to untying enough plot threads for the series to continue.
The Other Darrin: The series retained almost all the cast from the radio series with two exceptions - David Dixon replaced Geoffrey McGivern as Ford because McGivern did not suit the role visually and Sandra Dickinson replaced Susan Sheridan as Trillian when she became unavailable.
The background characters in the restaurant at the end of the universe are equipped almost entirely with costumes recycled from earlier episodes, and in particular from the various Guide entries, thus helping justify the expense of items which otherwise would have appeared on screen for only a few seconds each.
A badge from one of the Golgafrincham hats would in future be worn by Arnold Rimmer.
Throw It In!: The scene with Arthur and Slartibartfast in the pod-vehicle was shot in a tunnel with overhead lights that unavoidably reflected in the pod's windshield. Someone on the production crew had the brilliant idea of pairing a science-fictionish sound effect to each reflection as it slowly slid into view, perfectly disguising their mundane origin.
Troubled Production: Douglas Adams described the creation of the series as "not a happy production. There was a personality clash between myself and the director. And between the cast and the director. And between the tea lady and the director." Said director, Alan Bell, puts the blame on Douglas, claiming they used to make lists of his ridiculous unfilmable ideas, to which Adams would reply that Bell "cheerfully admits he will say what suits him rather than what happens to be the case. And therefore there's no point in arguing." John Lloyd, the producer and co-writer of the radio show, was annoyed that he was made "associate producer" (he felt that the fact his credit literally explodes in the ending credits was a comment on how meaningless it was) and thought Bell was too concerned with getting things done efficiently, rather than getting them done right. The second series simply didn't happen: Adams wouldn't do it without Lloyd or Geoffrey Perkins; Bell wouldn't do it with them. It was suggested that Perkins could be script editor (since this would minimise his interaction with Bell), and he viewed the possibility of trying to wring scripts out of Adams under these conditions with horror. Adams then suggested replacing Bell with Pennant Roberts, who had directed several of his scripts on Doctor Who, but this was declined on the grounds that a writer having any say in the choice of director (or, for that matter, a drama director handling what was classed as a sitcom) simply wasn't done in those days. Nobody would back down, so...
What Could Have Been: Infocom planned to create a sequel, which would naturally have been based on the next book in the series. Douglas Adams lacked the time to be involved in making the game, so instead he recommended an author friend of his who he believed could fill the spot. Unfortunately, that did not work out. The game's designer on Infocom's side summed up why things fell apart as, "He's a good writer, but he can't wrap his head around how games work."