How to show “teaser” posts on blog / archive pages, otherwise full posts

You do not need to explicitly call the post-single template, this is something wordpress is doing automagically for you based on the URL structure. For archive URLs the archive template will be “called” while for single posts the single template will be “called. There are fullbacks with the ultimate one is the index.php template.

For a better understanding of the structure and fallbacks look at the template hierarchy