You can do that by having one form field hidden somewhere in the html and when ever user clicks on the commect clone the form element and append it to the comment div
checkout this jsFiddle example below is the code snippet
<div class="comment">
comment1
</div>
<div class="comment">
comment2
</div>
<div style="display:none">
<form id="commentForm">
<textarea name="comment"></textarea>
</form>
</div>
var Comment = {
init: function() {
$(".comment").bind('click', $.proxy(this.handleClick, this));
},
handleClick: function(evt) {
var form = $('#commentForm').clone();
var target = $(evt.target);
var isFormAvailable = $("#commentForm", target).length > 0;
if(!isFormAvailable) {
$(evt.target).append(form);
}
}
};
$(function() {
Comment.init();
});