When the only person who gets to speak with realistic diction is the brilliant but quirky scientist. Maybe you want to show that the character in question thinks so fast they they have to Motor Mouth to try to keep up. To be less charitable to the actors and writers, maybe it's what perfectly sensible Clever Stuff sounds like when you don't understand most of the words.
This character may also use characteristic phrases of Buffy Speak. The reason that most characters don't speak like this is because Realistic Diction Is Unrealistic.
If they're using a lot of big words, it's Sesquipedalian Loquaciousness. If they're tossing out swear words or slang just as frequently, they're Sophisticated as Hell. The opposite of this, where a character speaks in highly-calculated terms, is Spock Speak.
- The page quote references the fact that Jeff Goldblum has this as his Characteristic Trope, in part because he played the Trope Codifier. He's often a Motor Mouth whether he's playing a scientist or not — but when he is playing someone who works in the sciences it's virtually guaranteed. The first character of that type he played was the developer of an artificial heart in 1981's Threshold, but the Trope Codifier is his Star-Making Role: Seth Brundle, the Doomed Protagonist of The Fly (1986) — a reclusive quantum physicist who has created teleportation technology. (This film also plays with the trope in that the character becomes even gabbier once he starts mutating into a Half-Human Hybrid of human and housefly.) Once he played "chaotician" Dr. Ian Malcolm in the Jurassic Park franchise, the trope became straight-up popularized. Nowadays, it's easier to list significant roles in which he isn't this:
- Nashville, only his third film, in which he plays a silent motorcyclist who appears periodically in the background and in transitions between scenes.
- In Into the Night, his character is an aerospace engineer but that is not important to the plot; moreover, he is suffering from chronic insomnia. Thus, his dialogue is delivered at a normal pace or slower.
- Earth Girls Are Easy, arguably because his character Learnt English from Watching Television and doesn't have much dialogue!
- The Life Aquatic, The Grand Budapest Hotel, and Isle of Dogs, in keeping with the general performance aesthetic of Wes Anderson films. It especially applies to the second, in which he plays a lawyer with a very precise method of speaking.
- Man of the Year, where he portrays a very fast-talking corrupt CEO's assistant. No stuttering, no tripping over words, and he's speaking about twice as quickly as he does anywhere else!
- In The Mountain, he plays a traveling doctor who performs lobotomies (the film is set in The '50s). He has a normally-paced, outright soothing manner of speech most of the time. When he's off the clock, unwinding and hitting on women, the pace picks up a bit and there are verbal pauses, suggesting he might have been this trope but old age and going to seed slowed him down.
- Angel: Fred, though she's good-natured enough to backpedal a bit in her speeches. Even when describing how she's about to kill someone!
- Daniel Faraday on Lost.
- Abby Sciuto on NCIS has a serious case of motor-mouth.
- Phoebe's scientist boyfriend David (played by Hank Azaria) on Friends.
- Ross, the most intellectual member of the main cast, also tends towards this (and is mocked mercilessly for it).
- On Doctor Who, The Tenth and Eleventh Doctors both do this a lot. Ten is more of the gibbery type, while Eleven does a variant where he very calmly rattles off his thoughts as he goes, with the occasional apparent non-sequitur thrown in.
- The First Doctor tended to do this, especially when William Hartnell, who was getting on in years, forgot his lines and had to adlib.
- Arguably Murdock from The A-Team. Much of his craziness is laced with some genuinely brilliant stuff. He frequently gets the rest of the team out of tight spots by singlehandedly performing impossible rescues, and all the while he's babbling on unintelligibly.....and not always in English.
- Gaius Baltar of Battlestar Galactica (2003). The Sanity Slippage didn't help.
- Many characters on AMC's Rubicon, especially Will, Miles, and Ed.
- Ladies and Gentlemen, from Criminal Minds, may I present to you, Dr. Spencer Reid and his amazing rapid-fire statistics! Generally, though, he has a bit of a stammer in his speech when he tries to be social, like talking to a girl.
- Dr. Daniel Jackson of Stargate SG-1. This is Lampshaded in the pilot to Stargate Atlantis when Daniel is starting to explain something to Jack, and Jack asks if this is the part where Daniel gets excited and starts talking real fast.
- In Stargate Atlantis, Dr. Rodney McKay is prone to this, being an Insufferable Genius.
- The pilot episode of Alphas gave everyone realistic diction, but the rest of the series usually reserved it for Dr. Rosen.
- Reginald Barclay of Star Trek: The Next Generation is almost always stuttering, both because he's naturally a Shrinking Violet and because his brain typically functions faster than his mouth.
- In Genius: The Transgression, higher Inspiration (i.e. stronger mad science) has a detriment in the form of Jabir, an effect that makes it harder for Geniuses to talk to mere mortals or even fellow Geniuses.
- Professor Mordin Solus does this constantly in Mass Effect 2 to the point where, at times, the player is given the option to interrupt him (two Paragon interrupts and a Renegade one in his first appearance).
- In Mass Effect 3, Mordin's patient calls him out on his rambling tendencies because they're interfering with her sleep.
- Johnny Powell, the not-all-together supernatural expert in The Darkness 2.
- Royce of the Camerata in Transistor. Brilliant scientist, knows more about the Transistor than anyone alive, managed to temporarily control the Process, but also tends to trail off in the middle of sentences and awkwardly pause to find words.
- Delbert in Treasure Planet is an astrophysicist and constantly stammers. Considering he's played by David Hyde-Pierce that's unsurprising, though (apparently the role was written for him).
- Rick from Rick and Morty. Granted, that show makes a point of averting Realistic Diction Is Unrealistic in general, but even then, Rick gibbers a lot more than anyone else, often stopping only in order to belch, drink or fall asleep.
- Mumbles on The Dick Tracy Show sounds like he swallowed a malfunctioning vocoder but he's quite the sharpie. He and Stooge Viller are holding the Retouchables (Hemlock Holmes' charges) for ransom, and he comes up with the idea to collect the ransom and rub them out. Stooge loves the idea.