A trick that I used for this site. Always wondered how to get a post summary on the frontpage, but the same paragraph in bold in the full story? Well, that’s exactly what I was l asking myself back then. However, it’s very easy. Read along.
The basic idea is: Regular font weight on the front page: 
Same summary, but bold when you click on (more…): 
How do you get this done?
1. Open up index.php. Replace <?php the_content(); ?> by: <?php the_excerpt(); ?> <a href="<?php the_permalink(); ?>" title="<?php the_title(); ?>">(more…)</a>.
2. Open up single.php. Find the Loop and before <?php the_content(); ?> line insert this: <strong> <?php the_excerpt(); ?></strong>.
3. You should type the summary in the "optional excerpt" field. This way you don’t have to manually split the text. That’s all!