If you just want to pass values entered by a user, you could have something like

<input type="text" name="N" size="35">
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and then in the action script use  $name=param('N') to collect it. I use hidden fields in a case where I may not be using $name imediately, but I want the script to print out new HTML for a page with a new form and I want $name to be available in that form. In that case I would store $name as the value in a hidden field and then when the new form is submitted the script can collect the value.
(For example I wrote a webmail client where at some point I use a script to print an HTML page with an email printed on the page. I store the message text and sender's email address in a form in hidden fields so I can click on a reply button and open up a reply window where the address appears in a textbox and the quoted email appears in a textarea. Then I can enter my reply in that same textarea and send off the reply with another script. The address and original email text are parsed and saved on the read page so they could be used on the reply page. Of course, there are various ways to do this sort of thing. You can save things by having them appear as values in various input widgets like textboxes, but sometimes you may not want them visible immediately.)
chas
(Update: Maybe I was unsure what you meant and was too long winded...should have just said that instead of using a string for the value, use for the value a variable which contains some info you have collected from the user. That's what I indicated in my first reply

<input type="hidden" name="thing" value="$thing">
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)