just show the field comment by clicking button

you can do this very easily with jQuery

simply wrap your comment form in a div and hide, add a button or a link and on his click event show the comment form so smoothing like this:

<a href="#" ID="Leave_a_comment">Leave a Comment</a>
<div id="comment_form_wrapper" style="display: none;">
    <?php comments_template( '', true ); ?>
</div>

<script type="text/javascript">
    $(document).ready(function() {
        $('#Leave_a_comment').click(function() {
            $('#comment_form_wrapper').show();
        });
    });
</script>