Peak is defined by Longacre as "a zone of turbulence"; it occurs at the climax, denouement, and sometimes the inciting incident of a narrative or narrative pericope. There is no one marker for it. Peak is signified by a shift in the proportion of use of a particular grammatical device in the surface structure. If, for example, the author has been using short sentences, he may start using long sentences in the peak. If he has been using long sentences, he may use short ones in the peak. If he has been using past tense in a story, he may switch to present tense in the peak (a common but not necessary device). In Greek narrative, this shift is from the aorist as the mainline tense marker to the present or imprefect.
Longacre categorizes kinds of peak devices as follows:
I suspect that word frequency (lexical choice) is another device that Longacre has overlooked. There is an increase in the usage of words found only once in Mark toward the climax (crucifixion) and denouement (resurrection).
I refer you to the major source on peak:
Longacre, Robert E. 1996. The grammar of discourse. Second Edition. New York and London: Plenum Press. Chapter 2, Section 3.Longacre's earlier work on peak is mainly in four items by him.
Longacre, Robert E. 1981. A spectrum and profile approach to discourse analysis. Text 1(4):337-59.Although peak is usually defined for narrative texts, it may be found in non-narrative texts as well to mark what may be called a "hortatory climax," an "expository climax," or in the case of letters, an "epistolary climax." For a discussion of how to apply statistical models to determine peak and use of peak in expository texts, see my book:
________. 1983. The grammar of discourse. New York: Plenum Press.
________. 1985. Discourse peak as a zone of turbulence. In J. Wirth (ed.), Beyond the sentence, 81-92. Ann Arbor, Mich.: Karome.
________. 1990. Storyline concerns and word order typology in East and West Africa. Studies in African Linguistics, Supplement 10:1-181.
Terry, Ralph Bruce. 1995. A discourse analysis of First Corinthians. Dallas, Texas: SIL. Chapter 5, Section 1For an ancient author on peak in Greek texts (although he doesn't call it peak), see Longinus On the sublime, chapters 23 to 29. "Among those changes which he discusses are the expansion of the singular into the plural to convey the idea of multitude (23.2-3), the contraction of the plural into the singular to give an effect of sublimity (chap. 24), the use of the present tense in narrating past time in order to increase vividness (chap. 25), the change of the person addressed from the whole audience to a single individual also to give a vivid effect (chap. 26), the use of the first person for one of the characters to show an outbreak of emotion (chap. 27), and the use of periphrasis or circumlocution to give the work a far richer note (chap. 28-29)" (Terry 1995: 119-120).