Modify comments form WordPress

functions.php

<?php
add_filter( 'comment_form_default_fields', 'wpse_62742_comment_placeholders' );

/**
 * Change default fields, add placeholder and change type attributes.
 *
 * @param  array $fields
 * @return array
 */
function wpse_62742_comment_placeholders( $fields )
{
    $fields['author'] = str_replace(
        '<input',
        '<input placeholder="Name*"',
        $fields['author']
    );
    $fields['email'] = str_replace(
        '<input id="email" name="email" type="email"',
        /* We use a proper type attribute to make use of the browser’s
         * validation, and to get the matching keyboard on smartphones.
         */
        '<input type="email" placeholder="Email*"  id="email" name="email"',
        $fields['email']
    );
    $fields['url'] = str_replace(
        '<input id="url" name="url" type="url"',
        // Again: a better 'type' attribute value.
        '<input placeholder="Website" id="url" name="url" type="url"',
        $fields['url']
    );
    return $fields;
}

/**
 * Changing the view for user viewing comment form
 */
function alter_comments_defaults( $defaults ) {
	global $post;

		$defaults = array(

			'title_reply'          => '<a name="add-a-comment"></a>' . __( 'leave a comment', 'thirdrock' ) . ' ',
			'cancel_reply_link'    => __( 'cancel#a#comment', 'thirdrock' ),
			'label_submit'         => __( 'submit', 'thirdrock' ),
			'comment_field' 		=> '<p class="comment-form-comment"><label for="comment">' .
			_x( 'comment#box#label', 'thirdrock' ) .
			' * </label><textarea id="comment" name="comment" cols="45" rows="4" aria-required="true" placeholder="Comment"></textarea></p>',
		);

		return $defaults;
}
add_filter( 'comment_form_defaults', 'alter_comments_defaults' );