diff --git b/core/lib/Drupal/Core/Entity/EntityViewBuilder.php a/core/lib/Drupal/Core/Entity/EntityViewBuilder.php
index 5231f3a..7550ad4 100644
--- b/core/lib/Drupal/Core/Entity/EntityViewBuilder.php
+++ a/core/lib/Drupal/Core/Entity/EntityViewBuilder.php
@@ -112,8 +112,9 @@ public function buildContent(array $entities, array $displays, $view_mode, $lang
     module_invoke_all('entity_prepare_view', $this->entityType, $entities, $displays, $view_mode);
 
     foreach ($entities as $entity) {
+      // The content property may be null if no hook_view implementation added
+      // renderable content before this point.
       $entity->content = (!empty($entity->content)) ? $entity->content : array();
-      // Remove previously built content, if exists.
       $entity->content += array(
         '#view_mode' => $view_mode,
       );
@@ -134,7 +135,7 @@ public function buildContent(array $entities, array $displays, $view_mode, $lang
    *
    * @return array
    */
-  protected function getBuildDefaults(EntityInterface $entity, $view_mode = 'full', $langcode) {
+  protected function getBuildDefaults(EntityInterface $entity, $view_mode, $langcode) {
     $that = $this;
     // Create a closure that uses the current execution context object as its
     // invocation object.
@@ -192,8 +193,12 @@ public function entityViewBuilderPreRender(array $elements) {
     \Drupal::moduleHandler()->invokeAll($view_hook, array($entity, $display, $view_mode, $langcode));
     \Drupal::moduleHandler()->invokeAll('entity_view', array($entity, $display, $view_mode, $langcode));
 
+    // Build field renderables.
     $this->buildContent(array($entity), array($bundle => $display), $view_mode, $langcode);
 
+    // Do not override $build = $elements because hook_view implementations
+    // are expected to add content, not alter it. For example, mymodule_view
+    // should not change the #theme key.
     $build += $entity->content;
     // We don't need duplicate rendering info in $entity->content.
     unset($entity->content);