The stage directions for this scene are of some interest. Shakespeare specifies “thunder and lightning” (both effects easily achievable in his day), and these provide the ominous mood for what transpires in the scene. Since Cicero greets Casca (“Good even, Casca”), it is more than likely that they enter from opposite sides of the stage, which provides theatrical/visual interest. (Had they been conversing already, their conversation “in media res,” then obviously they would have entered together from the same side of the stage).
Notably Cicero has few words, a reflection of his character. Casca is voluble and reveals aspects of his somewhat cowardly, superstitious nature. Shakespeare is, however, also using Casca to provide verbal scene painting beyond that achieved by the thunder and lightning aforementioned. There has been a “sway of the earth,” and a very strong “tempest” (that Casca describes extensively and as unlike any other he’s experienced). Moreover, Casca has seen a slave with a burning hand, a wandering lion, “a hundred ghastly women” and their tale of more burning men, a night owl shrieking in daylight, etc. He ends his descriptions with the superstitious conclusion: “ I believe they are portentous things / Unto the climate they point upon.” However, and this is his function (to provide the strong element of rationality), Cicero cooly declares:
“Indeed it is a strange-disposèd time.
But men may construe things after their fashion,
Clean from the purpose of the things themselves.”
Indeed, that is a universal truism–whatever the evidence, people tend to interpret it subjectively.
It is also very tempting to hear Shakespeare’s own thoughts behind those words. He may have been brought up in rural Warwickshire where “things go bump in the night,” and where locals might tell tales of Robin Goodfellow and his supernatural pranks, but to put straightforward logic into a character’s mouth (that controverts strongly superstition) seems to require a parallel innate belief on the part of the author. But, then again, may be not–and while biographical interpretations of literature have their place (see Charles Dickens and some of his novels) they are also inherently dangerous?
Check out The Shakespeare Diaries.