Too Obvious?

When I'm driving, Anthony often warns me, "Don't crash!" (I'm not really that bad of a driver!) Recently, on a walk, we saw a roofer up on top of someone's roof. Anthony shouted, "Careful! Don't fall!" (I don't think the roofer heard him, but fortunately, he didn't fall).

These warnings seem a little obvious, but when to state something and when not to is a question I struggle with as a writer. I struggled with it as a college freshman as I left off what I thought were obvious conclusions only to be told that I needed to make them. I struggle with it now with my novel. What can I expect people to reasonably understand and what needs to be explained? You don't want to spend time boring your readers by explaining the obvious, but you don't want to make it incomprehensible either. And it's tricky, because what's obvious and what's not obvious depends on your background and experience and you don't know the experiences of all of your readers, although I guess that relates to knowing your audience as well.