CEOs only answer to shareholders insofar as they're expected to deliver a profit, which is actually very easy to do because most business "profit" is actually rent-seeking derived from resources (including intangibles) the business already owns. You have to be incredibly hubristic to fail as a corporate CEO. It's an easy job.
That's not true for small companies. Startups are harder, and startup CEOs are basically middle management within the "hive company" of venture capital. The true executives are the VCs who float from project to project, careers and fates diversified, and can peel their names off anything that looks like it's going to fail.
In big companies, though, most CEOs would do better not to show up, except once a quarter to give a speech or something. All they do is fire people, producing churn, and get in the way of the people who do the actual work. They can only fail if their egos get the better of them and they get so hubristic that they break something that used to be working. The same is mostly true of executives in general, although they have a little more vulnerability insofar as (a) if their bosses screw up, they may end up eating blame, and (b) if the CEO screws up so bad he himself gets fired, they'll probably also be replaced during the regime change.