"Diction" refers to the general style of language used by writers. Diction selections choose among the alternative expressions for a single idea. For example, if a writer wants to say, "Stephanie picked up a rock," rock may be expressed as a stone, pebble, Ignacius rock, sandstone, granite, chink of rock, gravel, sedimentary rock, etc. Diction determines the synonym selection for "rock."
Diction is comprised of vocabulary choices in four standard diction levels: (1) formal diction, (2) informal diction, (3) colloquial and (3) slang dictions. Daily conversation among educated people is most often in informal diction. Slang diction is usually used among lower class specialty groups who don't adhere to Standard language forms (this is called negative prestige by sociolinguists). One particular of diction vocabulary selection for writers is the choice between abstract and concrete, emotive or non-emotive, general or specific words.
"The Open Window" is written in the diction of well educated men and women in the 1800s and may be called informal diction. While Saki's diction may sound "formal" to contemporary readers, it is the diction of daily conversation among this class of English people in this historic era and is therefore informal. Formal diction is the diction of academic essay writing, science writing, technical writing and such as that.
Saki's informal diction features an interesting combination of abstract and specific words by employing (1) emotive, (2) abstract, and (3) specific words.
Word examples are:
Emotive words: endeavored ... succession of total strangers; dreadful
Abstract words: amusing; mental excitement
Specific words: little brown spaniel; talk; prospects for duck
Examples from Dialogue
Emotive: "I almost get a creepy feeling that they will all walk in through that window--"
Abstract: "I hope Vera has been amusing you?" she said.
"She has been very interesting," said Framton.
Specific: "his white waterproof coat over his arm, and Ronnie, her youngest brother, singing 'Bertie, why do you bound?' as he always did to tease her, ..."
This choice of highly educated though informal diction, with its mix of abstract and specific, creates a mood of eerie suspense and horror mixed with hopeful disbelief.