There's more advice about opening lines than any other sentence in fiction. Most of it amounts to the same instruction: create intrigue, drop the reader in the middle of action. As if the problem with a bad first line is that it isn't loud enough.
The Opening Line Makes a Promise About the Kind of Reading Experience This Will Be
Jane Austen's "It is a truth universally acknowledged, that a single man in possession of a good fortune must be in want of a wife" promises irony and social observation. You know before the first character appears that this book will be sharp and wry, and that class and marriage will run through everything. The sentence is doing something more interesting than hooking you. It's calibrating your attention. It's telling you to read with one eyebrow raised.
Kafka's "Someone must have slandered Josef K., for one morning, without having done anything wrong, he was arrested" promises bewildering institutional injustice. The passive voice, the resigned logic of "must have," the strange calmness of a man reasoning through his own arrest. These lines don't introduce the story so much as they tell you how to read it.
That's the thing most advice about opening lines misses. The reader needs to know what kind of attention to bring. A thriller opening asks for different reading muscles than a literary novel. The first sentence sets the frequency, and everything after either holds that signal or loses it. When you write your opening line, you're calibrating how the reader will care, not whether they will.
Voice Shows Up in the First Sentence or Arrives Too Late to Matter
Daphne du Maurier's "Last night I dreamt I went to Manderley again." In twelve words: the narrator has lost something, is looking back from a distance, and the house matters more than any person will. The voice arrives before the character does. You don't know her name or her circumstances. But you know her. You know her because of the word "again," which carries the weight of obsession and longing all at once.
Compare that to an opening line that's just action: "She ran through the forest." Nothing about who she is, what she's running from emotionally. It's a camera angle with no photographer behind it. Voice shows up in word choice, in the gap between what's said and what's felt. A character who opens by noticing the weather is different from one who opens by noticing what nobody else in the room seems to notice. Voice shows up in that first choice of what to see.
I'm not entirely sure voice can be manufactured. I think it appears, or doesn't appear, in the first sentence you write when you stop trying to sound like a writer. The moment you stop performing, you start revealing. And what you reveal, accidentally, in that unguarded first line is usually more interesting than anything you could have engineered.