<?php
$cast_member = get_field('cast_member');
if ($cast_member) {
?>
<span>Cast</span>:
<span>
<?php
$persons = array();
foreach ($cast_member as $person) {
$persons[] = '<a href="'.get_permalink($person->ID ).'">'.
get_the_title( $person->ID ).'</a>';
}
?>
</span>
<?php
echo implode(', ', $persons);
}