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 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!: Say it with me, people. It's "The Answer to the Ultimate Question of Life, the Universe and Everything". Not the meaning of life. 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."
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.
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.
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.
Author Existence Failure: Douglas Adams died a year before the film was completed, though after he'd finished his parts of the script.
God Created Canon Foreigner: Many of the differences from previous versions were Adams creations, from pre-death versions of the script, including the POV Ray, the flyswatters, Humma Kavula, and the romantic elements.
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.
Casting Gag: The Tertiary, Quandary and Quintessential Phases had a lot of cameos by actors who had featured in non-radio versions of the story:
Chris Langham, Arthur Dent from the 1979 Institute of Contemporary Arts stage production of Hitchhiker's, pops up as Prak in the finale of the Tertiary Phase.
David Dixon, the TV Ford Prefect, had a cameo in the second episode of the Quandary Phase, getting pissed off at Arthur for trying to donate to save the dolphins when he should know they've all vanished. (This case is lampshaded, as Arthur—still played by Simon Jones, who acted alongside Dixon in the TV series—asks if they've met before.)
Stephen Fry, the film's Guide, cameos in the third episode of the Quandary Phase as Murray Bost Henson.
The Quintessential Phase series had Sandra Dickinson, Trillian in the TV version, voice Tricia McMillan (Trillian's alternate universe counterpart). She reprises her TV role as Trillian herself in the Hexagonal Phase since she and Tricia merged at the end of the Quintessential Phase. Susan Sheridan's death in 2015 probably also factored into things.
In a non-remake-related example, Geoffrey Perkins, Douglas Adams's boss at the BBC, plays Arthur Dent's boss at the BBC in the second episode of the Quandary Phase.
Hipgnosis: Did the cover art for the British releases of the soundtrack albums.
In Memoriam: The first episode of the Hexagonal Phase is dedicated to Susan Sheridan, who had voiced Trillian in the previous radio series.
Milestone Celebration: The first episode of the Hexagonal Phase was broadcast on 8 March, 2018 — exactly 40 years after the broadcast of the first episode of the Primary Phase.
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. Likewise, there was a gap of 13 years between the Quintessential Phase (2005) and the Hexagonal Phase (2018).
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 she'd typed this name a dozen times and he simply said, "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 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 be worn by Arnold Rimmer.
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.
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...