You already got the post_object, you dont need to setup_postdata and in some functions you have to pass the id as integer, not a function (get_field)…
So you can echo directly from the post_object, something like this:
<?php if( have_rows('cast') ): ?>
<ul class="events">
<?php while ( have_rows('cast') ) : the_row(); ?>
<li>
<?php $post_object = get_sub_field('choose_cast_members'); ?>
<?php if( $post_object ): ?>
<?php
$thumbid = get_post_thumbnail_id($post_object->ID);
$thumb = wp_get_attachment_image_src( $thumbid ), 'thumbnail' );
$url = $thumb['0'];
?>
<?php //$post = $post_object; setup_postdata( $post_object ); //dont need to setup_post_data ?>
<a href="<?php echo get_permalink($post_object->ID);//here ?>">
<img src="<?php echo $url; ?>" alt="<?php echo $post_object->post_title; //here ?>" />
</a>
<h3><?php echo $post_object->post_title; ?></h3>
<?php echo apply_filters('the_content', $post_object->post_content); ?>
<?php //wp_reset_postdata(); ?>
<?php endif; ?>
</li>
<?php endwhile; ?>
</ul>
<?php endif; ?>
