The only readily available way for a GM to portray significant NPCs in a distinctive manner is to give them easily identifiable speech patterns. Obviously, our GM employs this method. In general, it's an effective method. If you can simply talk without introduction and your players can instantly recognise which NPC is addressing them, that's a good thing.
But it can cause unintended consequences if the voices get out of hand or you have to switch between them too rapidly. We suspect Billy West would make a great GM.
GM: You spy the Trade Federation leader who was banished from Naboo talking to Count Dookû, a retired Jedi Master.
Nute Gunray: My Rord Dookû, why should the Fedelation join your cause?
Count Dookû: Sacrebleu! Ah am telling you, Palpatine, 'e eez up to somezing.
Obi-Wan: Why is he talking in a French accent?
GM: It's just to indicate that he's from a different culture.
GM: He's not French French, he's—
R2-D2: Space French.
GM: ... Yeah.
GM: They head into a meeting room. What are you doing next?
Obi-Wan: I stay hidden and follow them.
Obi-Wan: I'm just going to sit here and spy on them for a while. Please continue.
Nute Gunray: And what, play tell, do you imagine the Chancerror is up to?
Count Dookû: Ah 'ave evidaunce! You weel soon compre-hende... mouzz-ai-... vous.
GM: It's not easy switching between these accents, you know.
Obi-Wan: Je comprends.