add_filter( 'wp_dropdown_users_args', 'change_user_dropdown', 10, 2 );

​

function change_user_dropdown( $query_args, $r ){

    // get screen object

    $screen = get_current_screen();

​

    // list users whose role is e.g. 'Editor' for 'post' post type

    if( $screen->post_type == 'post' ):

        $query_args['role'] = array('Editor');

​

        // unset default role 

        unset( $query_args['who'] );

    endif;

​

    // list users whose role is e.g. 'Administrator' for 'page' post type

    if( $screen->post_type == 'page' ):

        $query_args['role'] = array('Administrator');

​

        // unset default role 

        unset( $query_args['who'] );

    endif;

​

    return $query_args;

}