How can I catch the current node object and make changes into it before it is rendered?
hook_node_view_alter

This hook is called after the content has been assembled in a structured array and may be used for doing processing which requires that the complete node content structure has been built.
If the module wishes to act on the rendered HTML of the node rather than the structured content array, it may use this hook to add a #post_render callback. Alternatively, it could also implement hook_preprocess_node(). See drupal_render() and theme() documentation respectively for details.