I got hooked by 'Fate/Zero' before I even knew there was a light novel, and when I finally picked up the book it felt like slipping into the same room but seeing the furniture rearranged. The most obvious difference for me is voice: the novel is drenched in internal monologue and authorial description. Scenes that the anime shows with a sweeping camera and pounding music are often replaced by long, intimate paragraphs in the book—especially when it's Kiritsugu or Kirei thinking. That means you get more of the characters' private justifications, doubts, and small memories that explain why they make such brutal choices, which made me sympathetic to some characters I never expected to like.
Visually, the anime turns the big set pieces into unforgettable spectacles, so it sometimes trims or condenses exposition to keep pacing. The novel, on the other hand, can afford slower beats: more political background, more detail about the Einzbern lab and the personal history that haunts people after the war. Little scenes exist only in one medium or the other; a throwaway paragraph in the book can be an entire silent shot in the show, and vice versa. Translation choices also matter—some of the philosophical lines land differently on the page than they do when an actor speaks them with music.
If you're the type who enjoys introspection and savoring language, the novel rewards you with layers. If you live for visceral battles, voice acting, and soundtrack highs, the anime will probably hit harder emotionally in the moment. I tend to flip between them depending on my mood: the book when I'm reading on a rainy afternoon and want to linger, the anime when I need that rush of visuals and sound to make a bored evening feel epic.