Now what if you have to add/remove fields from default contact form to change the feel of your comment box? I am eliminating website field from default comment box by playing with ‘fields’ argument:in comments.php
<?php $comment_args = array('title_reply' => 'Got Something To Say:',
'fields' => apply_filters('comment_form_default_fields', array(
'author' => '<p class="comment-form-author">' . '<label for="author">' . __('Your Good Name') . '</label> ' . ($req ? '<span>*</span>' : '') .
'<input id="author" name="author" type="text" value="' . esc_attr($commenter['comment_author']) . '" size="30"' . $aria_req . ' /></p>',
'email' => '<p class="comment-form-email">' .
'<label for="email">' . __('Your Email Please') . '</label> ' .
($req ? '<span>*</span>' : '') .
'<input id="email" name="email" type="text" value="' . esc_attr($commenter['comment_author_email']) . '" size="30"' . $aria_req . ' />' . '</p>',
'url' => '')),
'comment_field' => '<p>' .
'<label for="comment">' . __('Let us know what you have to say:') . '</label>' .
'<textarea id="comment" name="comment" cols="45" rows="8" aria-required="true"></textarea>' .
'</p>',
'comment_notes_after' => '',
);
comment_form($comment_args); ?>