Gramp: Walter Matthau
Sheldon: Billy West
Dante: Bob Denver (ala Maynard G. Krebs) Arthur: Hank Azaria
Oso: Nika Futterman
Flaco: Peter O'Toole

Sheldon=Jay Baruchel
Arthur=Jeff Anderson
Flaco=Speed Levitch
Dante=Seth Rogen
Grampaw=William Shatner

If we're picking old actors from the John Wayne crowd, I'd pick Victor McLaglen over Barry Fitz (no accents for Gramps, please).

I think three suggestions made for Dante were also acceptable: Seth Rogen, Adam Corolla, and Christian Slater.

While I still like Wilford, I figured out an even better voice, the ideal Gramps. He does have a subtle accent, but it's American--the guy who does the Whataburger commercials, William H. Bassett. He even looks kind of Gramp-ish (if a bit skinny).

See here for a pic: http://www.tbatechnology.com/guardianangel/news.as p

Too bad Burl Ives and Mr. Whipple's aren't still around. They'd be good, too.

...must...scroll...quickly...avoid...influencing.. .choices...

in no particular order...

Flaco - Issac Hayes (South Park's Chef)
Sheldon - Patton Oswalt
Dante - young Michael Keaton as Johnny Kelly in Johnny Dangerously
Gramp - a composite of Wilford Brimley and Captain Kangaroo
Arthur - Dennis Miller (defining sample : "HA! Frank, ya maroon!")

(ok, these are all lies...I hear myself at varying ages for all of them but Flaco, and lately he's sporting a British accent in my head, but that's not my fault - they play those commercials SO MUCH.)

Never posted before, Dave, so I gotta say this - you are AWESOME!

I always thought Arthur would sound like Gilbert Gottfried.

I have only two.

Since listening to the podcast I have always thought of Dave as the voice of Arthur. His voice quality and inflection, especially when he's riffing, is perfect for the little white duck.

As for Flaco, who better to voice him than Baltimore Ravens quarterback Joe Flacco?

Or is that one too obvious?

Sheldon: Eddie Murphy

Gramps: Eddie Murphy

Arthur: Eddie Murphy

Flaco: Eddie Murphy

.
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Dante: Arsenio Hall

Almost forgot!

Oso: Dan Aykroyd

Gramps: Edward Asner
Arthur: Jay Leno
Dante: Adam Rodriguez
Sheldon: Billy Boyd
Falco: Jewel Staite (I know Falco is a guy, but you got to go for that really hi pitched "SQEEE!")

Actually I've always envisioned Flaco being voiced by Mark Hamill. Voice of the Joker :) great laugh and high enough pitched to really give those Squee's some substance!

My friend and I were just discussing how cool this would be a movie or series.

Gramp: Derek Jacobi
Sheldon: Mark Hamill
Arthur: Brent Spiner
Dante: Dave Foley
Flaco: Since you already described his voice in comic as high pitch:
Michael Winslow,
Rosearik Rikki Simons (the voice of Gir),
or possibly Flea (voice of Donnie from The Wild Thornberry's)

sheldon: lee pace
gramp: sir ian mcKellen
arthur: adam west
dante: jason lee
flaco: the fantastic mr. eddie izzard

I don't have a good list of voices, but want to offer a tangent:

It would be cool if webcomics like Sheldon were offered in an MP3 format. "WHAT?!" you say, but hear me out:

Get your voice actors together, record each panel as a separate MP3. Make the panel art the "album art" for the MP3s. Make a playlist for the MP3s which covers all of the panels.

Et voila! You have a webcomic you can bring to your iPod or whatever, without worrying about how the lettering in the word balloons is going to be destroyed by shrinking the comic to the 320 x 240 screen size.

Grandpa: Malcolm McDowell. Not really sure why...but there it is. On second though, perhaps Hugh Laurie. With the American accent.

Sheldon: Wil Wheaton. Done.

Dante: I think Brent Spiner may be able to do something here.

Arthur: I am completely agreeing with going with NPH for this. DO IT!

Flaco: Robin Williams. Do you realize just how much he'd be able to do with that?

Can you tell I'm a ST fan?

Oh, @mgmdrums nailed Flaco. I suggested Peter MacNicol as a regular, with frequent guest stars. But Robin Williams could do all of those voices all by himself!

I have to agree with Vivi in the very first post on some of the voices:

Gramp: Walter Matthau. As soon as I saw that name I knew it had to be him. He both had the humour and the grumpyness that's so essential for this part.

Arthur: Hank Azaria. I'd also love to hear what he could do with it! :-D

Flaco: James Earl Jones! Definately! Reading that almost cost me a new keyboard and screen. (I had a mouthfull of soda when I read it!) If it has to be high pitched then I'd vote for Rowan Atkinson.

Sheldon: Fred Savage. His voice from "The Wonder Years". I think that would be credible.

Dante: I'm stumped on this one....

Oso: Hank Azaria again.