Blazer Six Blog: Comment Preview Post (http://www.blazersix.com/blog/treasure-hunting-in-wordpress-core/)

comment-preview.js

<script type="text/javascript">
jQuery(function( $ ) {
	$( '#comment-preview-button' ).on( 'click', function( e ) {
		e.preventDefault();
		
		// The fields won't be present for logged in users.
		var commentAuthor = commentForm.find( 'input[name="author"]' ),
			commentAuthorEmail = commentForm.find(' input[name="email"]' ),
			commentAuthorUrl = commentForm.find( 'input[name="url"]' );
		
		// Do client side validation of comment data before performing the AJAX request.
		$.ajax({
			url : 'wp-comments-post.php',
			type : 'POST',
			data : {
				mode : 'preview',
				author : commentAuthor.length ? commentAuthor.val() : '',
				email : commentAuthorEmail.length ? commentAuthorEmail.val() : '',
				url : commentAuthorUrl.lenght ? commentAuthorUrl.val() : '',
				comment : commentForm.find( 'textarea[name="comment"]' ).val(),
				comment_post_ID : commentForm.find( 'input[name="comment_post_ID"]' ).val()
			},
			dataType : 'json',
			success : function( data ) {
				// Insert the comment HTML.
			}
		});
	});
});
</script>

comment-preview.php

<?php
/**
 * Filter requests for comment previews and return JSON.
 *
 * @param mixed $approved Approval status (0|1|'spam').
 * @param array $commentdata Comment information.
 * @return mixed Prints comment data if context is preview, otherwise returns approval status.
 */
function blazersix_comment_preview( $approve, $commentdata ) {
	// Check to make sure the context is 'preview'.
	if ( isset( $_POST['mode'] ) && 'preview' == $_POST['mode'] ) {
		// Retrieve custom HTML here.
		$commentdata['html'] = blazersix_get_comment_html( $commentdata );
		
		// Short-circuit the request and return comment data.
		echo json_encode( $commentdata );
		wp_die();
	}
	
	return $approve;	
}
add_filter( 'pre_comment_approved', 'blazersix_comment_preview', 10, 2 );