In the play their reason for leaving him out is very different:
He will never follow anything
That other men begin.
(II. i. 151.)
It seems to me, however, highly probable that Shakespeare had read the Life of Cicero and obtained his general impression from it, though he invents the particular traits. The irritable vanity and self-consciousness of the man, which Brutus’ objection implies, are, for example, prominent features in Plutarch’s portrait. So too is his aversion for Caesar and Caesarism, which makes him view the offer of the crown, abortive though it has been, as a personal offence: Brutus observes that he
Looks with such ferret and such fiery eyes
As we have seen him in the Capitol
Being cross’d in conference with some senators.
(I. ii. 186.)
But he is very cautious, and even when venting his vexation in one of those biting gibes to which, by Plutarch’s statement, he was too prone, he takes care to veil it in the safe obscurity of a foreign language. “He spoke Greek ... but those that understood him smiled at one another and shook their heads” (i. ii. 282). This has sometimes been misinterpreted. Shakespeare has been taxed with the absurdity of making Cicero deliver a Greek speech in a popular assemblage. Surely he does nothing of the kind. It is a sally that he intends for his friends, and he takes the fit means for keeping it to them; much as St. John might talk French, if he wished to be intelligible only to those who had made the Grand Tour and so were in a manner of his own set. Plutarch lays stress on his familiarity with Greek, as also on his study of the Greek Philosophers. This may have left some trace in the description of his bearing in contrast to Casca’s, when they meet in the storm. Cool and sceptical, he cannot guess the cause of Casca’s alarm. Even when the horrors of earthquake, wind and lightning, are described in detail, he asks unmoved: