Please note that I am by no means an expert, and this is my view entirely. I could be quite wrong. Nevertheless, the following is what I think could have been improved.
I was surprised to find that the start of Tomorrow, When the War Began was quite slow. It wasn’t until Chapter 6 when we really dived into the action; a quarter of the way through the book. Marsden spends the first chapters developing characters, the setting, and placing clues. These clues start off slow and increase in number as we get closer and closer to the big reveal of the war. Though I appreciate the slow build of tension, having the inciting event so late is not typical of young adult novels, as teenagers tend to have shorter attention spans and appreciate more action early on (note this is a generality and never bothered me as a teenager. In fact, I was apt to consume whatever book was in the house during my teenage years).
It was interesting to see at first Marsden had a little difficulty in attributing traits to his secondary characters (or at least I believe he did). I can see why they swapped some of the characters lines and activities around in the film. For example, in the book Homer is the first to find the snake “Jesus Christ!…There’s a snake in my sleeping bag!” I can forgive Homer for his brief lapse in courage; I’m pretty sure anyone would react like that to finding something writhing in their bed, but it’s Kevin, the coward, who initially steps forward (with Robyn) to get the snake out. Kevin’s courage is relatively short-lived and he soon freaks out, but I can understand why they changed this in the movie, and made Kevin the one with the snake in the sleeping bag, and Homer as the brave man to fix the situation. Not to say the film did a great job in developing the characters. The film went almost too far in the opposite direction, adhering to stereotypes as a cheap and dirty method to build the characters, which is ultimately why it failed to go large. Anyhow, there were a few other instances too (in the book) where I thought, I don’t think [this person] would say [that]. But, Marsden soon ironed out his characters, and by the sixth chapter, he seems to have a good handle on them.
I think one of his other weaknesses would have to be chapter beginnings and ends, but these do also get better as the book increases pace, and I’ve noted in the next few books of the series he gets more adept at this. Chapter 3 ended with them going to sleep (which, if you’re reading in bed seems an excellent place for you to yawn and go to sleep too). The start of the next chapter wasn’t exactly attention grabbing either, beginning with the sentence “We didn’t do a lot the next day.” They were far from perfect, but on average, like the ending of one of his other chapters, I’d have to say “…it sounded OK to me. Not great, but OK.”