Caesar's observations about Cassius are not just about his physical appearance but also about his character traits, which he finds troubling. Caesar describes Cassius as having a "lean and hungry look," indicating a man who is not content and is always thinking, which he perceives as dangerous. Cassius's lack of interest in frivolous activities like plays and music, and his tendency to smile in a way that seems mocking or scornful, make it seem like Caesar sees Cassius as a man who is never truly at ease, especially when he sees someone greater than himself, making him a potential threat.