Actor-Inspired Element: Jim was written to be British but Cillian Murphy struggled to do an accent - so they let him use his natural voice. London has a high number of Irish immigrants anyway.
Naomie Harris worked with Danny Boyle to come up with a backstory for Selena's cynical attitude; they decided that Selena had to kill her whole family in one afternoon. Knowing this explains why she is so heartbroken for Hannah after Frank is infected and her speech to Jim about their family. It also adds another layer onto her being so protective of Hannah in the latter part of the movie.
What Could Have Been: The original script had a completely different second half, where the focus remains squarely on the four survivors as they hole up in the lab that started the whole mess. It was replaced by the story with the soldiers because of a problematic plot element of replacing every single drop of blood in the human body. Essentially Jim would offer to save Frank and the scientist would cure him by swapping their blood around.