In Act 2, Scene 1, as Brutus delivers his soliloquy and shares his plans to kill Caesar with the audience, he uses an idiom to describe his predicament:
It is the bright day that brings forth the adder,
And that craves wary walking.
Just as one ought to tread carefully on sunny days, when snakes might emerge to bask in the light, Brutus surmises it may be best to stay wary about Caesar while things seem to be going so well for him. By itself, the phrase "it is the bright day that brings forth the adder" may not seem to mean much, but in context—and in conjunction with Brutus's eventual characterization of Caesar as a serpent egg waiting to hatch—the idiomatic function of the sentence becomes clear. Brutus follows up with another idiom right away:
Crown him that,
And then I grant we put a sting in him
That at his will he may do a danger with.
To "put a sting in" Caesar is to arm him with the power to, as Brutus says, "do a danger with," or do something dangerous. Yet again, Brutus resorts to the language of a venomous snake—and the "sting" of a snake's fangs—to characterize Caesar as a cunning, evil creature.
Shakespeare's plays are known for their interlocking literary devices, running jokes, and constant wordplay. The addition of idiomatic expressions into the mix, some of which are idiomatic to the Early Modern English of Shakespeare's time and no longer make sense in Modern English, increases the interpretive wealth of Shakespeare's dialogue but makes it more challenging to fully apprehend as an audience member. In this case, Brutus's reliance on snake imagery to characterize Caesar gives him a clearer perspective on the threat that Caesar could pose to the Roman Republic—and ultimately emboldens Brutus to join the conspiracy to kill him. As Shakespeare explores the power of language in Julius Caesar, manners of speech become matters of life and death.
