Or can you maybe do both?
I've heard so much conflicting advice about this (in general, not directed at my work) and it's making me have doubts about my opening for my WIP (action). I'm prone to doubts anyway. And the idea that readers/agents/publishers often don't read past the first page freaks me out. After awhile a story's scenes become pretty set (flowing one from another), but at the beginning there are so many possibilities it's overwhelming. Any thoughts / advice?
You've correctly identified this as a self-confidence issue rather than a storytelling issue.
My suggested fix: Gather your 5 favorite novels in the genre you're writing in. Read each opening scene. Do they start with action? Show the character? Show the character in action? Hint at or flat out provide the main story goal?
As you reread those opening scenes, try to soak in the confidence of your favorite authors. They trust that their story is good, that the reader has an attention span longer than a fruit fly's, that as long as the writing flows and
something happens, the reader is going to stick with it. That's all you really need.
Don't try to second-guess what will grab a slush reader or agent by the nutsack. Even
they can't articulate what hooks them--they can only complain about what bores them. Avoid the "jaw-dropping" opener that falls flat, doesn't deliver, doesn't fit the story, or is dishonest (the wild action scene that turns out to be "just a dream," for example).
Simply start with a person (character) in a place (setting) with a problem (immediate goal). As Kurt Vonnegut once said, every character has to want something, even if it's just a glass of water. Don't be afraid to make that initial goal small in relation to your overall plot. Have confidence in your story and trust in your readers that it will be enough.
Most importantly, until you've completed the first draft of your manuscript all the way to the end, do not put too much thought into the opening scene. Where you begin is important, but the right place to begin isn't always clear until you know how the story ends.