Required Fields in a block?

Question

I am creating a sign in sheet using Livecycle.  I want to make certain fields required if the user starts filling in that block of fields.
This is what it looks like:
First Name
Last Name
Email
Degree
Address
City, State, Zip
First Name
Last Name
Email
Degree
Address
City, State, Zip
First Name
Last Name
Email
Degree
Address
City, State, Zip
If someone fills in there first name, is there a way to require them to fill in their last name and email address?
When I have tried required fields it makes them required for the entire sheet, not that one record.
Thanks

Answer 1

Jono,
I think I am headed in the right direction thanks to your help.
I added the code and changed it for my fields but it does not seem to be triggering.
topmostSubform.Page1.FirstName::exit - (JavaScript, client)
if (!(this.isNull) { //if field isn't null
          LastName.mandatory = "error";
          LicenseType.mandatory = "error";
          //etc
else { //if field is null
          LastName.mandatory = "disabled";
          LicenseType.mandatory = "disabled";
          //etc