Initial values in comments

I’m trying to make is so some initial text shows up in the comment boxes if the visitor has not been to the sight yet.

I tried the code below in the comments.php file, but it didn’t work. I don’t know too much about php, I could be going about it wrong.

Any ideas?

Thanks, Nathan

<?php if ( $comment_author ) : ?>
	<input type="text" name="author" id="author" class="textarea" value="<?php echo $comment_author; ?>" size="28" tabindex="1" />
<?php else : ?>
	<input type="text" name="author" id="author" class="textarea" value="John Doe" size="28" tabindex="1" />
<?php endif; ?>