Assigned someone accountability for an outcome (e.g., improved sales, on-time delivery, enhanced team morale) can set an expectation that might or might not be accepted.

However, if someone takes accountability on their own accord, that is by taking the initiative, the potential for success is much higher.

Remember what you perceive as acceptance of accountability might actually be just simple acquiescence or even worse passive-aggressive behavior (appearing compliant while actually not in agreement at all).

Accountable Behavior

When someone accepts accountability, it is usually very apparent through their behavior.   So much so, I have observed situations where someone is assumed to be accountable just because of how they are acting, even though they are not the assigned accountable person.

In these cases, the accountable resource could be so lethargic or apathetic  relative to executing their responsibilities, the person who takes charge (on their own volition) is just assumed to be accountable.

Believing you are accountable is much more powerful than being told you are.  Self-motivated people do not need organizational power to act.  Their behavior speaks louder than their role.

Empowerment

Strong leaders create an environment of empowerment to nourish self accountability.  Few restrictions on activities encourage you to take on more.

Taking ownership and getting things done can be invigorating.  Feeling empowered to do it just removes a barrier to action that many of us want to undertake anyway.

So get off your but, take charge and make it happen.  JUST DO IT.