"Now, I am the first to appreciate a joke," said Arthur and then had to wait for the others to stop laughing.
Deadpan Snarker: Clearest and strongest in the original radio drama, where his sarcasm actually makes him famous among the bird people of bird people of Brontitall. In other versions this side of him is usually downplayed quite a bit, but he still occasionally gets in a few sarcasms.
Ford: How would you react if I said I'm not from Guildford after all, but from a small planet somewhere in the vicinity of Betelgeuse? Arthur: I don't know. Why, do you think it's the sort of thing you're likely to say?
Cheshire Cat Grin: Smiles "a little too broadly, giving people the unnerving impression he was about to go for their neck."
Creepy Blue Eyes: In the TV series. The actor tried using purple contacts but they were just gilding the lily of his already creepily intense eyes. (In And Another Thing... it's revealed that Ford finds it relaxing to not blink and can go for eight minutes without doing so — he's timed it and wonders if it's a new record.)
Deadpan Snarker: Actual verbal irony is not a concept they have on his planet, but he still manages to be a smart-ass without it.
Multiple Head Case: Subverted in most adaptations (where there's no distinction between the two heads), pretty much played straight in the movie, and zigzagged in the sixth novel, where the second head has a distinct personality after being removed and attached to the Heart of Gold.
Screwy Squirrel: He is a complete scoundrel, will almost screw anything over for personal gain, and FAR from a role model. Practically the reason why he is the president of the galaxy in the first place, actually.
That Man Is Dead: In the backstory he lobotomised himself to keep his plans secret even from himself. However, turns out that the 'new him' hates the old one and actively works against those plans.
Tricia MacMillan/Trillian Astra
Radio: Susan Sheridan; TV: Sandra Dickinson; Film: Zooey Deschanel
Ambiguously Brown: In the books, where she's "darkish", with black hair and brown eyes and a "vaguely Arabic" appearance when she wears a headscarf, although her actual ethnicity is never mentioned.
Radio: Stephen Moore; TV: voiced by Stephen Moore, body of David Learner; Film: voiced by Alan Rickman, body of Warwick Davis
Catch Phrase: "I think you ought to know I'm feeling very depressed", "Life? Don't talk to me about life." some of the first two lines he says when introduced in every continuity of H2H2. There's also his "Here I am, brain the size of a planet..." speeches.
The Chew Toy: He literally gets treated like crap (or makes himself believe he is, at times) by almost everything in the entire universe. The fact that he is several times older than the universe itself in the later books doesn't help much, either.
Flawed Prototype: Marvin was the unsuccessful prototype for the emotion chip. Everything else that has it is irrepressibly cheerful all the time— including Eddie, a ship AI who will cheerfully tell you you're about to be vaporized by nuclear missiles, and even the individual doors which all thank you for passing through them. Marvin hates them all.
Image Song: He had four such songs sung by Stephen Moore, his actor from the original radio and TV series.
Ridiculously Human Robot: Subverted. While humanoid, his emotion chip is supposed to emulate real emotions. Unfortunately, it does that too well, and only with depression.
Robot Buddy: Under certain definitions of the word "buddy". The marketing division of the Sirius Cybernetics Corporation was probably referring to a different robot when it advertised "Your Plastic Pal Who's Fun To Be With!"
At one point he is put in charge of all the computing for a robotic army and fleet that can take on the whole galaxy at once, and is still so under-challenged that he composes little poems to keep himself occupied.
Talking the Monster to Death: In one instance, Marvin defeats a sentient armoured tank by asking it to guess what weapon he has been given with which to defend Zaphod against it. When he reveals that the answer is "Nothing", the tank gets so angry that it blasts out the floor, causing it to plummet to its destruction.
In the first book, he kills two policemen on Magrathea by plugging up to the ship controlling their life support system and having a long chat, explaining to it his opinion and perspectives of the Universe...
Ford: And what happened? Marvin: It commited suicide.
A lot later, amongst the swamps of Sqornshellous Zeta, he recalled to the matress Zem the time he, some miles away and five million years earlier, had opened a bridge meant to revitalize the economy of the Sqornshellous System (after the entire economy of the Sqornshellous System had been spent building the bridge) with a depressing speech in which he expressed his spiteful contempt for everyone who would ever cross the bridge in question and then plugged into the bridge's opening circuits.
Zem: Voon, and was it a magnificent occasion? Marvin: Reasonably magnificent. The entire thousand-mile-long bridge spontaneously folded up its glittering spans and sank weeping into the mire, taking everybody with it.
Time Abyss: Oh so very, very much. In one instance he stays in one spot from approximately 1980 untill the end of the universe. By the end of the series he is, by virtue of Time Travel, six times older than the universe itself! ...He is then brought back to life again because numerous characters that lived when he was created were still alive, and that went against the Lifetime Insurance Policy of Sirius Cybernetics Corporation.
"The first ten million years were the worst. And the second ten million, they were the worst, too. The third ten million I didn't enjoy at all. After that, I went into a bit of a decline."
Radio: Richard Vernon (first and second series), Richard Griffiths (fourth, fifth, and sixth); TV: Richard Vernon; Film: Bill Nighy
Noodle Implements: How Wowbagger became immortal involves a rubber band, a boxed lunch, and a high-powered particle accelerator. People who have attempted to recreate the circumstances that made him immortal have failed, looking very stupid (or very dead) in the process. Or both.