[I.236] The idea seems to be that, as men start a huge fire with worthless straws or shavings, so Cæsar is using the degenerate Romans of the time to set the whole world a-blaze with his own glory. Cassius's enthusiastic hatred of "the mightiest Julius" is irresistibly delightful. For a good hater is the next best thing to a true friend; and Cassius's honest gushing malice is surely better than Brutus's stabbing sentimentalism.
[I.237] The meaning is, Perhaps you will go and tell Cæsar all I have said about him, and then he will call me to account for it. Very well; go tell him; and let him do his worst. I care not.
[I.238] Fleering. This word of Scandinavian origin seems to unite the senses of 'grinning,' 'flattering' (see Love's Labour's Lost, V, ii, 109, and Ben Jonson's "fawn and fleer" in Volpone, III, i, 20), and 'sneering,' and so is just the right epithet for a telltale, who flatters you into saying that of another which you ought not to say, and then mocks you by going to that other and telling what you have said.
[I.239] Hold, my hand: stay! here is my hand. As men clasp hands in sealing a bargain. In Rowe's text the comma is omitted.
[I.240] Be factious: be active. Or it may mean, 'form a party,' 'join a conspiracy.'
[I.241] griefs: grievances. The effect put for the cause. A common Shakespearian metonymy. Cf. III, ii, 211; IV, ii, 42, 46.
[I.242] undergo: undertake. So in 2 Henry IV, I, iii, 54; The Winter's Tale, II, iii, 164; IV, iv, 554.
[I.243] by this: by this time. So in King Lear, IV, vi, 45.
[I.244] Pompey's porch. This was a spacious adjunct to the huge theater that Pompey had built in the Campus Martius, outside of the city proper; and there, as Plutarch says in Marcus Brutus, "was set up the image of Pompey, which the city had made and consecrated in honour of him, when he did beautify that part of the city with the theatre he built, with divers porches about it." Here it was that Cæsar was stabbed to death; and though Shakespeare transfers the assassination to the Capitol, he makes Cæsar's blood stain the statue of Pompey. See [III, ii, 187, 188].
[I.245] element: sky. Twice Shakespeare seems to poke fun at the way in which the Elizabethans overdid the use of 'element' in this sense, in Twelfth Night, III, i, 65, and in 2 Henry IV, IV, iii, 58.