Preview: Some fields are not filled with the "Fill Form" option

And, how do you fill in the "blanks"---when I do to "preview" I can't seem to fill in the blanks..when does this happen? also, signing it??? that doesn't work with the preview either.  Do I need to publish to see if these things work?

Please note that some form fields cannot be filled with the "Fill Form" option. 

Only the Signature field in the "Fill In Blank" field cannot be prepopulated with the "Fill Form" option. 

The best way to check if the form is working properly is to open it in a new browser tab and manually fill it.