Radio buttons on the form

[seeya98]

When placing radio button on the form I can make it do something when user clicks on it. Let`s say I can make some form field "hide" when I click on radio button. But how do I make an opposite action? I want form field to be hidden by default and show up when user clicks on radio button. Apparently "show" command should be used, but how do I make form field invisible or hidden by default?