Ugh I just bought Still Life not realizing it's a sequel to Post Mortem. Can you play Still Life and fully understand the story, background and characters without having played Post Mortem?
Yes, you can.
I've played it in the same way. Although some characters mention the events from Post Mortem, Still Life has a story in its own right. There are no points where you will be confused if you haven't played the prequel.