We played Call of Cthulhu the other day--in the default '20s, and in London and a cruise to London--so that means I had to do a lot of voices. Luckily, I have a lot of voices.
You've probably heard the Goblin--which is just a slightly-lower-pitched munchkin, really--it's achieved by making the tongue as wide as possible and pulling it to the back of the top teeth. The words have to be articulated mostly by the lips.
The Gollum, in contrast, is accomplished by compacting the tongue, laying it on the bottom of the mouth, and pulling it as far back as possible.
Widening the tongue, flattening it on the bottom of your mouth, and pulling it as far back as possible gives you a sort of Orc, or at least a grumbly sort of monster.
(All of this is accompanied by some mysterious movements back in the larynx which are harder to describe. The best I can say is that high voices seem to involve pulling things in your neck higher up and lower voices involves pushing them lower down.)
With most accents, I'm a decent mimic just after I've heard the voice--which is a cruel thing to be good at, since it means you're best at copying someone while they're standing right in front of you. (With a non-foreign voice it's much harder--I'm not really imitating the voice--that's very hard--I'm imitating the accent, which is enough for those who don't share the accent. Imitating a voice well enough that people with the same accent like it is rare freakshow talent.)
The trick isn't really getting the accent, it's remembering the accent. This is easy with people you've heard talk a lot--I was taught for a semester by the Indian photographer Raghubir Singh
and, after he died, one of his (American) friends (another photography teacher) kept insisting I do his voice whenever we talked because, she insisted, it made her miss him less--which was strange because Raghubir and I despised each other and most of my Ragubhir routine consisted of making fun of how he discussed photography using tennis analogies.
Another one I've got fairly down is whatever kind of British accent the writer Martin Amis has--I've heard him a lot in interviews and readings--and I can do a decent rip of one of my close friends' Spanish accent--though if you've never heard him it just sort of sounds like an unplaceably generic Eurovoice with lisps and extra e's before any s-word.
For most others, I need a "trigger" phrase--something I can say that tells me where all the parts of the mouth go. Examples:
For the Irish, I use the phrase "Ian Alistair McKenzie" from the name of an addressee in an old UPS or FedEx ad commercial. One "Ian Alistair McKenzie" and I can be convincingly Irish to the non-Irish for days at a time.
For the Cockney, I use the phrase "This is King's Cross"--an actual quote from an actual cabbie in actual London that I actually had said to me.
For the West Indian I use "'Dere no a creme for dat" another actual quote from an actual Londoner said to me in a cosmetics shop. It still needs some work, as my Jamaican tends to slide (bizarrely) into my Irish if I talk long enough. It doesn't come up much in D&D though.
My Liverpool is just Ringo saying "I'm just a chap from Liverpool" in an old commercial from the '80s for credit cards or some shit. (The other Beatles are easy once you've got that--John is that with your eyes half-closed and longer O's, Paul is that but moving your head back and forth and more excitable, George is that but lower and bored.)
Johnny Rotten, Sid Vicious, or anybody from Python is cake, but they're so extreme that it ends up just sounding like the NPC is Johnny Rotten, Sid Vicious, or The Minister of Silly Walks, so for a more generic British voice I use whoever it is that says "The Tale of Sir Robin".
I find a softened Johnny Rotten works ok for Melniboneans and White Elves, despite the fact that, classwise, it makes no sense.
For the Japanese, I just need to remember hearing a kid in Tokyo say his favorite band was "Arrannashid" and then having him write it down and so seeing he was saying "Rancid." Though I fear that on bad days I just sound like the Trade Guild.
For the German (the non-funny German, for the funny German I just do Colonel Klink like everyone else) I use my friend Werner saying "It's kind of weird." But I drift out of it very fast if I'm not in Germany. And when I am in Germany it insults the Germans, so it's tough. I can do Schwarzeneggar, Hans and Franz and Rainier Wolfcastle well enough to tell them apart, but I kind of hate that voice so I avoid it.
For French, I use the name "Lemme Caution" as pronounced in Alphaville.
Russian is Yakoff Smirnov saying "Een Sovyet Yoonion," but Russian tough-guys are harder, and my Russian-tough-guy tends to slide right into my Henry Kissinger unless I remember someone my friend Dave once got in a fight in a bar saying "I am not afraid off you, it was not for your police, I would hit you." but then that actually slides into a soup of Eastern European accents I heard when I made a movie in Spain.
Nordic accents are hard--I lived in Denmark for a month, and not one of them sounded a bit like the Swedish chef--they all had learned English from the BBC and had what sounds to an American like a merely slightly hesitant and less crisp British newscaster accent. Any attempt to do a more "Nordic" sounding accent just descends into Swedish Chef. How D&D vikings should sound is beyond me--I usually just say Fuck It and go for some sort of Rider of Rohan Melange.
For American children, I remember my little brother used to pronounce "Are you thirsty?" as "Aw you sauce D?"
Many of my American accents are weird, since, arguably, I actually have some of them and it's more a question of getting into the emotional mindset that goes with the accent then remembering a phrase. My native DC-area accent (best exemplified by Omar on The Wire) is a kind of Southern-plus-what-most-people-think-of-as-"black" an it's sometimes how I actually sound when I get angry, and it's actually hard for me to do it unless I'm actually making threats. The key component is a sort of constant oscillation between incredulity at what the other person is saying and confident indifference to whether they believe what you're saying. "I'on'even care, I tell you what, you wanna believe the motherfucker, you believe the motherfucker. I'm'onna sit back and drink mah motherfucka' applejuice. Be right here if you neeme."
Most of the Americans don't come up in RPGs I play, though, so whatever.
Like in "Axe" Episode 2, I don't always like doing female voices--if you do them funny, it's funny, and if you're a guy and manage to nail it and don't do them funny, it can be even funnier--and sometimes there's enough funny business going on that you don't want more funny. Sometimes.
But anyway I have some good ones:
For the female New York Jewish, I use my aunt saying "She went from Poe-lind ta Choina! She was at Hu-RO-shima!" (Long story. Not sure it's true.) and "Theya's no excuse fuh that kyna behaveyuh." and, simply, "For her" pronounced as "Fuh huh."
For the south, I use "hotter'n Georgia asphalt" as pronounced by Laura Dern in Wild at Heart.
For Female Russian (ironically better than my male Russian), I can do my friend Vera saying "I would like some wudka, and a cookie."
For the female French I use the phrase "Je n'aime pas les tunnels" from, I think, the movie "Night On Earth".
For what I assume is upper middle British I use "I'm Catty Kay for the BBC".
For the upper class British I use Margaret Thatcher saying "Let's have a party! Acid party! Rave! Rave! Rave! Murder." as sampled in the song "Maggie's Last Party".