Would you say its best to get into the action as soon as possible or just wait and slowly build it up?

You need to add a touch of mystery from the word go in order to keep the readers interest. Sometimes action can give away the mystery and if it is introduced too soon then reading the story will become boring. Add the action where you want to by all means but leave an air of mystery so that the reader will want to continue reading to find out what is going to happen. There is an argument for building suspense and also one for introducing action fairly quickly but the trick is not to give too much of the plot away too soon. Good luck with your story.