When Caesar tells Antony
Let me have men about me that are fat;
Sleek-headed men and such as sleep o' nights:
Yond Cassius has a lean and hungry look;
He thinks too much: such men are dangerous....
This is not a literary device. Shakespeare is borrowing directly from Plutarch, as he did for much of the material in his play. In Plutarch's Life of Julius Caesar
Caesar said: "I am not much in fear of these fat, long-haired fellows, but rather of those pale, thin ones," meaning Brutus and Cassius.
Evidently Brutus in real life was also thin, but Shakespeare chose to apply the description only to Cassius. Throughout the play Cassius is characterized as a miser. It is interesting to consider whether real misers and misers in literature, such as Charles Dickens' Ebeneezer Scrooge, are typically lean and hungry-looking. There would be a good reason to think so. A miser loves money and hates to part with money. Therefore he might eat and drink as little as possible because he would dislike spending money on food and wine even for his own consumption. There are some interesting examples of the miserliness Shakespeare attributes to Cassius in the play.
In Act 1, Scene 2 when Cassius invites Casca to supper, Casca declines, claiming, probably falsely, that he has a prior engagement. Casca obviously doesn't like Cassius. Cassius is not a likable man, which is why he is so anxious to get Brutus to act as the leader of his conspiracy. Brutus is a kind, generous man and well liked by everyone. Cassius is not well liked because people sense that he is cold, selfish, greedy, and miserly.
Cassius first invites Casca to "supper," which is a light meal taken in the evening. He is thinking that he can give Casca a little bread and cheese along with some inferior wine. When Casca declines, Cassius ups the ante, as follows:
Will you dine with me to-morrow?
Ay, if I be alive and your mind hold and your dinner
worth the eating.
These men have known each other since childhood. Casca knows that kind of meal he can expect at the home of Cassius, where the slaves are probably all lean and hungry-looking too. Note that Cassius apparently thinks that a "dinner" will be more tempting to Casca than a "supper." He judges Casca by himself, as most people are inclined to do. He is anxious, of course, to get Casca alone where he can sound him out about joining his conspiracy against Caesar. Casca accepts the invitation rudely and reluctantly. He knows that if he refuses the second invitation Cassius will persist in his invitations and he will either have to accept one or else openly insult Cassius by telling him he doesn't like him and doesn't want to sup or dine with him.
Misers are dreadful people. They are like black holes that take in everything and give out nothing. The Bible cautions against having anything to do with them in Proverbs 23.
6 Do not eat the bread of a miser, nor desire his delicacies;
7 For as he thinks in his heart, so is he. "Eat and drink!" he says to you, but his heart is not with you.
8 The morsel you have eaten, you will vomit up, and waste your pleasant words.
Brutus made a mistake in listening to Cassius and trusting him. Cassius was bad luck for all the conspirators, who were either killed in battle or massacred by Antony, Octavius and Lepidus in the purge that followed the formation of the new triumvirate.