"Glad to meet you, sir. I have heard your name mentioned in connection with that of your friend. You interest me very much, Mr. Holmes. I had hardly expected so dolichocephalic a skull or such well-marked supra-orbital development. Would you have any objection to my running my finger along your parietal fissure? A cast of your skull, sir, until the original is available, would be an ornament to any anthropological museum. It is not my intention to be fulsome, but I confess that I covet your skull."
To which Holmes astutely responds:
"You are an enthusiast in your line of thought, I perceive, sir, as I am in mine," said he. "I observe from your forefinger that you make your own cigarettes. Have no hesitation in lighting one."
Holmes helps the reader feel like a smartie here, by stating the obvious: Mortimer is an "enthusiast" (Watson prefers the term "strange")--an obsessive, in other words.
This leads me to the point of this week's reading. On a web site I recently visited, one agent said she particularly looks for stories in which the dialog is vivid and distinctive: that is, all the characters should not sound the same. I think this aspect of dialog-writing often gets overlooked; we get hung up on what the characters are saying, and making sure they say it in the least boring way possible. But, although all these guys all come from a single source--the author's brain--they must appear to reflect their own separate and distinctive development as human beings. For example, Mortimer is a phrenology nut--which means his language is peppered with the language of his profession, and also reflects the rhythms of obsession: his stated attempt not to be "fulsome" suggests he's been criticized for ranting on and on about skulls before, but his preemptive apologies only reveal his interest more strongly.
In other words, to be convincing and distinctive, characters need to use language in ways that reflect their particular physical and emotional circumstances. A doctor is going to use precise anatomical language, which means that if we are creating a doctor character, we have to become familiar with such language and the ways in which that particular doctor would deploy it. She might speak in a notably detached way about, say, the progress of cancer in a lung. Or, for a different effect, she might speak in peculiarly loving detail about it, unsettling her interlocutors and readers. Or she might convey an unusual capacity for empathy by constantly checking to see if her patient understands her terms. The point is, because the doctor is this particular doctor, she isn't going to sound like that other doctor over there. Mortimer and Watson are both men of science (Mortimer says he's a mere "dabbler,") but they don't talk the same way. On the other hand, neither Mortimer nor Watson is going to use a term like "that lumpy thingie on your head"; their scientific backgrounds enable them to know the contemporary, technical language for it.
See what I mean? This is not to say you should lard up your characters' dialog with "distinctive" (i.e. distracting) verbal tics and professional jargon. But it is worth thinking about different people you know in real life, and how they all express themselves a little differently, even if they're in the same profession. And if they're from different professions and backgrounds, their speech should differ accordingly, if still subtly. Finally, speech reflects temperament. If someone's impatient, don't just tell us that: have him speak impatiently.