it depends on the return value for the post object field.
If you are returning a post object then you can’t just echo the value, if you want, for example to display the post title
$object = get_field('film_realisatrice', $post->id);
echo get_the_title($object->ID);
On the other hand if you are returning the id then you could do something like
echo get_the_title(get_field('film_realisatrice', $post->ID));
