Half-Life2.net have a quick interview with Marc Laidlaw, science fiction author and story writer/designer for Valve on the Half-Life games.
Asked how the episodes have changed the way they develop the plots, he explains,
"They forced our hand...instead of jumping forward another 10 years and letting players fill in the gaps for themselves, we've gotten the chance to develop some of the elements deliberately. But we've also had the chance to spend more time working on our characters and letting more of the plot emerge from them and their dynamics, rather than simply out of the demands of saving the world as quickly as possible."
Read the rest of it here. (Warning: it contains some fairly major spoilers early on).