When Shaun and the group are running out of Liz's flat they are all carrying weapons of some kind, but only Shaun actually hits any zombies. This was because only the cricket bat that Shaun was carrying was a padded fake, all the other items were real and would have hurt the extras playing zombies if they had been hit with them.
According to writer-director Edgar Wright in the DVD commentary, when Ed attempts to cheer Shaun up at the Winchester with plans of binge drinking, he is actually summarizing the events of the next day (Z-day) entirely in drinking references. "Bloody Mary" - Checkout Girl, "Bite at the king's head" - Phillip, "Couple" - David and Di, "Little Princess" - Liz, "Stagger back" - impersonate zombies, "Bar For Shots" - firing rifle.