Responsibility

What am I responsible for? I have a job description that says I am suppose to do my job. Sometimes I notice that others who do not work for me but work with me may not be doing their job correctly. Do I have a responsibility to correct it? If I notice it after they started doing it wrong, am I responsible to tell them? Am I responsible for not telling them earlier? If I notice something, it is right for me to tell them, but it might not be my responsibility. If it is right to do something, am I responsible to do it? This is why I can't sleep at night.