I don’t have comments.php… how do I customize my comment fields

A little late, but you could also add the following to your theme’s functions.php file:

    /** Comment Form Function Defaults */
    add_filter('comment_form_defaults','my_comment_defaults');
    function my_comment_defaults($defaults) {
        global $user_identity, $id;
        if ( isset($post_id) )
            $id = $post_id;
        else
            $post_id = $id;
        $commenter = wp_get_current_commenter();
        $req = get_option( 'require_name_email' );
        $aria_req = ( $req ? " aria-required='true'" : '' );
        $fields =  array(
            'author' => '<div class="comment-form-author">' . '<label for="author">' . __( 'Name:' ) . ( $req ? ' <span class="required">*</span>' : '' ) .
                        '</label><input id="author" name="author" type="text" value="' . esc_attr( $commenter['comment_author'] ) . '" size="30"' . $aria_req . ' /></div>',
            'email'  => '<div class="comment-form-email"><label for="email">' . __( 'Email:' ) . ( $req ? ' <span class="required">*</span>' : '' ) .
                        '</label><input id="email" name="email" type="text" value="' . esc_attr(  $commenter['comment_author_email'] ) . '" size="30"' . $aria_req . ' /></div>',
            'url'    => '',
        );
        $required_text = sprintf( ' ' . __('Required fields are marked %s'), '<span class="required">*</span>' );
        $defaults = array(
            'fields'               => apply_filters( 'comment_form_default_fields', $fields ),
            'comment_field'        => '<div class="comment-form-comment"><label for="comment">' . _x( 'Comment:', 'noun' ) . ' <span class="required">*</span></label><textarea id="comment" name="comment" cols="45" rows="8" aria-required="true"></textarea></div>',
            'must_log_in'          => '<p class="must-log-in">' .  sprintf( __( 'You must be <a href="https://wordpress.stackexchange.com/questions/81950/%s">logged in</a> to post a comment.' ), wp_login_url( apply_filters( 'the_permalink', get_permalink( $post_id ) ) ) ) . '</p>',
            'logged_in_as'         => '<p class="logged-in-as">' . sprintf( __( 'Logged in as <a href="%1$s">%2$s</a>. <a href="%3$s" title="Log out of this account">Log out?</a>' ), admin_url( 'profile.php' ), $user_identity, wp_logout_url( apply_filters( 'the_permalink', get_permalink( $post_id ) ) ) ) . '</p>',
            'comment_notes_before' => '<p class="comment-notes"><em>Note: </em>' . __( 'Your email address will not be published.' ) . ( $req ? $required_text : '' ) . '</p>',
            'comment_notes_after'  => '<p class="form-allowed-tags">' . sprintf( __( 'You may use these <abbr title="HyperText Markup Language">HTML</abbr> tags and attributes: %s' ), ' <code>' . allowed_tags() . '</code>' ) . '</p>',
            'id_form'              => 'commentform',
            'id_submit'            => 'submit',
            'title_reply'          => __( 'Leave a Reply' ),
            'title_reply_to'       => __( 'Leave a Reply to %s' ),
            'cancel_reply_link'    => __( 'Cancel reply' ),
            'label_submit'         => __( 'Post Comment' ),
        );
        return $defaults;
    }

This could be a bit of overkill, but it also will give you control over the other fields and attributes of the comment form.