Customise Comment form

The default comment form fields are defined like so:

$fields =  array(

  'author' =>
    '<p class="comment-form-author"><label for="author">' . __( 'Name', 'domainreference' ) . '</label> ' .
    ( $req ? '<span class="required">*</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">' . __( 'Email', 'domainreference' ) . '</label> ' .
    ( $req ? '<span class="required">*</span>' : '' ) .
    '<input id="email" name="email" type="text" value="' . esc_attr(  $commenter['comment_author_email'] ) .
    '" size="30"' . $aria_req . ' /></p>',

  'url' =>
    '<p class="comment-form-url"><label for="url">' . __( 'Website', 'domainreference' ) . '</label>' .
    '<input id="url" name="url" type="text" value="' . esc_attr( $commenter['comment_author_url'] ) .
    '" size="30" /></p>',
);

And they are passed through a filter:

'fields' => apply_filters( 'comment_form_default_fields', $fields )

So to modify them, you can just add a filter:

function wpse126157_comment_form_fields( $fields ) {
    // Your code here

    // Return something
    return $fields;
}
add_filter( 'comment_form_default_fields', 'comment_form_default_fields' );

Note the important information in the Codex:

Note: To use the variables present in the above code in a custom
callback function, you must first set these variables within your
callback using:

$commenter = wp_get_current_commenter();
$req = get_option( 'require_name_email' );
$aria_req = ( $req ? " aria-required='true'" : '' );

So you can modify your callback accordingly:

function wpse126157_comment_form_fields( $fields ) {

    // Include these if you intend to use them
    $commenter = wp_get_current_commenter();
    $req = get_option( 'require_name_email' );
    $aria_req = ( $req ? " aria-required='true'" : '' );

    // Your code here

    // Return something
    return $fields;
}
add_filter( 'comment_form_default_fields', 'comment_form_default_fields' );