What are examples of diction in Act 5, Scene 3, lines 55-90?

"Diction" simply means the choice of words used by an author. Good choices of diction will use words appropriate to the context, or producing a certain effect. We'd normally need to identify what kind of diction we are looking for in order to give examples of it (informal, formal, slang, colloquialisms, etc.). Looking at this passage from Julius Caesar, the diction varies between conversational, informal diction and highly formal aureate diction. For example, Titinius's "He lies not like the living" and Messala's reply ("Is not that he?") is an informal exchange, written in prose. The passage from Messala that follows, however, is an example of formal diction, written in blank verse (iambic pentameter) and using vivid imagery to indicate that Cassius's death symbolizes the end of Rome: "our day is gone."
After Messala has exited the scene, after line 83, we see a still better example of this formal and aureate diction in Titinius's appeal to dead Cassius, concluding as it does with a rhyming couplet. In Shakespeare, we often see rhyming couplets used to mark significant events or endings: here, it precedes Titinius's suicide.