I am trying to figure out whether, or not, it is possible to add a comment to the field ("cell") of the form created in LiveCycle Designer (similar to the function "Add a Comment" available in Excel, when you can hide a comment and on touch of a cursor information appears).
The scripting language used in the form was FormCalc.
Here's my take: I gather you want the "fillee" of the form to be able to clearify his or her answer. The only option without doing scripting is to add an extra field (which could be expandable). If you're familiar with form scripting, however, you could for instance do any of the following:
Add a link (icon or text) that popups an interactive message box (for comments).
Add a link (icon or text) that expands an extra comment field.
(And just in case I misunderstood you, and you want the "fillee" to be able to add Acrobat comments, open the (static PDF) form in Acrobat Pro and choose Comments->Enable for commenting and analysis in Adobe Reader...)