Drupal module to hide part of body field initially
You can do this using a jQuery plugin. See this drupal/stackoverflow response.

The jquery project page for readmore. Other jquery modules can do this as well such as Expander. Expander looks newer and more supported (off hand).

You could just add this file to your site's theme folder and theme info file. You probably want to not use Teasers anywhere and just rely on the JS readmore links for a CSS selector targeting your node body content. Your theme may have a more specific selector.