Example:

I'm creating custom fields in the proposals module. I want to create a field called Proposal Source, with a picklist of options. One of the options will be "requested by manager." 

If that option, and only that option, is selected, I would like another field to appear that is a checkbox for "manager received city notice." If the option "requested by manager" is not selected, then I want the checkbox to be greyed out or not visible. 

Is there a way to do this?