It might be interesting if, rather than merely speaking through the raven, he has to telepathically communicate with the raven and hope it says what he wants it to. In other words, treat the necromancer as genuinely unable to speak and have the raven as an independently-willed 'translator', who sometimes accidentally-on-purpose gets it wrong or even says rude/inappropriate things without any input from its 'master' (leaving the poor necromancer to glare at it with a "WTF you stupid bird, you're going to get us both killed!" sort of look).
Alternatively, perhaps the raven is actually the real necromancer, whose soul is trapped inside a bird due to him screwing up the ritual that should have made him immortal (his soul went into his familiar rather than his phylactery?). The 'wizard' is just a convenient peasant/corpse/slave/mentally-handicapped-identical-twin the necromancer has mind-controlled (or paid) and dresses up in his old robes so nobody realises what's happened (because the ritual that went wrong is highly illegal).