The more general rule is that all actions a character takes during its turn have to be executed in order. They can't overlap. This only applies to the normal actions, minor, move, and standard. Other types of actions have their own rules and often do happen in the middle of other things.
Note that FREE actions (apparently) can also happen at any time. So a warden can move 3 squares, mark enemies adjacent, and then keep moving because applying his mark is a free action. He couldn't move 3 squares, attack, and move 3 more because that would be overlapping normal actions.