Once your actualy in a game go to options and go under the game tab. You'll see two buttons (twoard the bottom) that control your hero behaviour.
One sets her/him on defensive ... ie if they are attacked they will autoattack back. This is nice because if you happen to ne looking elsewhere your hero wont just stand there and die ;p
One sets her/him on aggressive ... which pretty much puts them in DS1 mode and they run around killing etc without much need for your input, though you can still control them directly if you want.
Also dont forget about the 2 autocast slots in your spellbook, no matter what mode your in, or if you have melee/ranged weapons active they will cast these spells as needed.
Myros