More or less chronological, but if I'm writing chapter 2 and suddenly am struck with inspiration for a scene in chapter 15 I'll jump ahead and write it.
This is more how I write. The story, to me, needs to be presented chronologically as much as possible. However, I don't just jump forward, but also backwards.
If something coming up is dominant in my mind, I'll write the first draft of the event. It gives me an idea of where to shape 'current' events. Since I'm normally more of a 'seat-of-my-pants' writer, this is pretty uncommon.
If I reach a point where I need a deeper understanding on something going on, however, I will also step back and explore what happened previously, even if it doesn't make it into the book. If I want a character to have an irrational fear of fire, I may have a general idea of why (tragic house fire killed his parents, for example).
When this fear becomes evident and another character is prompted to ask about it, suddenly I wonder how the phobic character would answer. So I go back and write a scene describing the fire, the surrounding events, and this character's place in it. This helps me picture his mindset about the event and his fear and I can have him react accordingly.
I have hundreds of these little mini-stories about characters, cultures, and religions I create for my projects.