Auto Excerpt Everywhere Plugin by Serena Villa
Great for use with theme Twenty Twelve
They say :
“Created to help those people who don’t like to edit code in template files (or are too lazy to do that). Just activate the plugin and it will automatically show the excerpts in your homepage, archives and search results.
It tries to display your custom excerpt text and if it doesn’t find it, it will show an automatically generated excerpt. You can also define excerpt length, custom read more link and attach a thumbnail.”
Oct 2015 Note : “This plugin hasn’t been updated in over 2 years. It may no longer be maintained or supported and may have compatibility issues when used with more recent versions of WordPress.”