A lot of people know what needs to be done but don't want to do what needs to be done. This is what prevents people from doing great things.
Whether it's because of a fear of failure, apathy, laziness, an unwillingness to relinquish control—or all of these things combined or some other reason—being a knower who isn't a doer can be dangerous territory to live in.
Just think about it for a second.
How many of us know that person—that person who can come up with a million reasons why that presentation, system, meeting, or project isn't as good as it should be, but they are completely unwilling to help do what needs to be done to make it better?
We all know that person; don't really like or want to be that person, but if we're being honest, all of us have been that person at one point or another—whether outwardly or inwardly—and we need to be conscious of how much this prevents excellent work from happening.
It's easier to know the next step, and harder to be the next step. But all of us need to know and be the next step. So we don't end up being that person.
I think about fighter jets, and how every person piloting them needs to know and do what needs to be done. Or else the consequences are catastrophic.