Poltava,
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:
(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...)