Nope, you can only use class-specific abilities when you're that class. Also, while you can change classes at will (at least once you've gone through the quests to obtain the mage and thief classes, as well as through the necessary dialogues with certain companions so that they can change your class wherever you are) each class levels up individually so you can't just gain a bunch of levels as a fighter then expect to switch to a mage and suddenly be able to cast high level spells. Also, if memory serves, stat increases only occur when you gain a level in your highest class (so if you've gained 10 fighter levels, then switch up to a mage, you'll only gain the next stat point once you hit level 11 as a mage). Additionally, there are two class bonuses ( I think at levels 7 and 14), but you'll only get both of them if you hit both those levels as the same class. Because of all of this, while it's still worthwhile to go through the quests to gain the ability to switch to the thief and mage classes, for the bulk of the game you'll want to try to stick to a single class rather than switching between classes on a regular basis.