Oh in that case you'll need to move those two lines out of the first loop.
$select_actors=mysql_query("SELECT * FROM actors_in_movies WHERE movie_ref='$movie_ref'");
if (mysql_num_rows($select_actors)>=1) {
while ($row=mysql_fetch_array($select_actors)) {
$actor_ref=$row['actor_ref'];
$select_actor_name=mysql_query("SELECT * FROM actors WHERE actors_ref='$actor_ref' AND active='1'");
while ($row_actor=mysql_fetch_array($select_actor_name)) {
$actor_name .= $row_actor['actors_name'] . ",";
}
}
$actor_name = substr(trim($actor_name), 0, -1);
echo " $actor_name";
}