Assigning keys to potions or spells, open the potion / spell list by the character's portrait, hover the mouse over what you want to assign a hotkey for and press one of the function keys.
In the options, I changed the default character selection keys to the number keys, to free up the function keys for spells or potions.
The selected character is controlled by you, even if set to aggressive. The non-selected character should attack automatically when set to aggressive.
