How do I get the output of a single field? How do I get the output of a field of a node?
I have some weird subpage construct using entity references (one 'main' node and a few 'sub'-nodes).

Each of these nodes have a header image to be displayed above the content. What I am trying to do is for the sub-nodes to 'inherit' the header image of the main node.

I implemented hook_node_view and I have got the node, I just don't know how to build the renderable array for it. 

function hook_node_view(array &amp;$build, \Drupal\Core\Entity\EntityInterface $entity, \Drupal\Core\Entity\Display\EntityViewDisplayInterface $display, $view_mode) {
  if($view_mode == 'full'){
    if((!isset($build['field_headerimage']) || !isset($build['field_headerimage'][0])) &amp;&amp; $entity-&gt;hasField('field_location')){
      $location = $entity-&gt;get('field_location')-&gt;referencedEntities();
      if(is_array($location) &amp;&amp; isset($location[0])) {
        $node = $location[0];
        dsm('yay, now what?');
        dsm($node-&gt;label());
      }
    }
  }
}
The answer to my own question seems to be like so:

$build['field_headerimage'] = $node-&gt;get('field_headerimage')-&gt;view();