I'm using the_excerpt() on index.php . Not in page.php nor anywhere else.
But when i click on my menu (generated by: wp_get_archives('type=postbypost&limit=15') ) my posts doens't display properly. The just show one excerpt (instead of them all, like in index.php)
Any comments will be helpfully.
When you click on a post title that is generated with the template tag, wp_get_archives(), then the Template Hierarchy will use the single post hierarchy to display that ONE post.
So determine what Template is displaying your single posts and make the change to that template to support display (or not displaying the |excerpt.
Related:
first four questions here: FAQ_Layout_and_Design#Text_and_Content_Display