In Julius Caesar, why does Brutus say "O Conspiracy" in Act 2 Scene i just before the conspirators arrive?
Upon hearing that his friends have arrived incognito, Brutus refers to them collectively as "the Faction," but then references them with the word "Conspiracie," to which he then attributes human facial characteristics. Since they've shown up, Brutus realizes they intend to follow him in killing Caesar; his plans can be carried out. However, the parallel here is that the men, traveling to Brutus's house, don't wish to be discovered, just as much as Brutus doesn't want the plot to assassinate Caesar to be discovered; to avoid detection, "Keep smiling." Note the chracteristics attributed to "Conspiracie" through the use of the words face, brow, visage, native semblance:
No, Sir, their Hats are pluckt about their Eares,
698: And halfe their Faces buried in their Cloakes,
699: That by no meanes I may discouer them,
700: By any marke of fauour.
Let 'em enter: They are the Faction. O Conspiracie,
703: Sham'st thou to shew thy dang'rous Brow by Night,
704: When euills are most free? O then, by day
705: Where wilt thou finde a Cauerne darke enough,
706: To maske thy monstrous Visage? Seek none Conspiracie,
707: Hide it in Smiles, and Affabilitie:
708: For if thou path thy natiue semblance on,
709: Not Erebus it selfe were dimme enough,
710: To hide thee from preuention.