Cosi funziona:
codice:
<html>
<head>
<title></title>
<script language="Javascript">
function showDiv(radioselect) { 
    document.getElementById("autEmail").style.visibility = 'hidden'; 
    document.getElementById("autSito").style.visibility = 'hidden'; 
    document.getElementById("autEmail").value = ''; 
    document.getElementById("autSito").value = ''; 
    document.getElementById(radioselect).style.visibility = 'visible'; 
    document.getElementById(radioselect).focus(); 
}
</script>
</head>
<body>

<form action="" name="form" method="post">
uno <input type="radio" value="autEmail" name="opzioniAutore" onClick="showDiv('autEmail');"><input type="text" style="visibility:hidden" id="autEmail" size="15" name="autEmail"">
due <input type="radio" value="autSito" name="opzioniAutore" onclick="showDiv('autSito');"><input type="text" style="visibility:hidden" id="autSito" size="25" name="autSito"">
tre <input type="radio" value="autTesto" name="opzioniAutore">

</form>
</body>
</html>