Just some random characters...
Cersei - Nicole Kidman, yes she's a few years older than Cersei, but she could be ice cold one minute and a conniving witch the next. I personally don't like NK, but I think she'd be great... they'd just need to pad her bra a bit.
Aemon - Jack Nicholson, I think Nicholson could steal every scene playing the blind maester of a destroyed house.
Greatjon - John Rhys-Davies, yes JRD can play the blustering blowhard to a T. If not him, then Brian Blessed... he played Voltan in Flash Gordon, which was essentially the same character. Let one of them be the Greatjon and the other can be Robert Baratheon.
Tywin - Ken Watanabe, I know he's Japanese, but his glare, his stoicism, and magnetism could work. Hey if Denzel Washington could play Don Pedro of Aragon in Much Ado About Nothing, why not KW? If you don't like him, then how about Kevin Kline? Just shave his head.
Baelish - Tim Roth, he's not physically big enough or good looking enough to be the Kingslayer (sorry, TK), but he's sly enough and nasty enough to be Littlefinger.
Tyrion - Daniel Craig, yes I know he's the new James Bond, but his looks don't strike me as, um, well, attractive. They shrunk Elijah Woods to be Frodo...
Olenna Tyrell - Louise Fletcher, they'd have to shrink her a bit, but she could be crusty... and nasty if you push too hard.
Jaime - Stephen Dorff, yes I know I complained that Tim Roth was too short for Jaime... and Dorff is short as well, but with blond hair he'll look like Jaime, they just need to put lifts in his shoes.
Dolorous Edd - Dan Akroyd, I can see Akroyd deadpanning all those one liners! Or if you think Akroyd is too old, then Ben Stiller could do it.
Roose Bolton - Al Pacino, 'nuff said.
Pycelle - Richard Harris, remember Harris' character in Unforgiven? He acts like a hard on, but he's soft. Or Brad Dourif might be even better.
Sandor - Russell Crowe.
Edmure - Chris O'Donnell, I don't really like him, but I think he'd be a natural as the empty headed playboy.
Asha - Bridget Moynahan.
Brienne - Vince Vaughn, seriously! He's tall enough and he'd make an ugly woman... and I think it's the kind of stretch as an actor that Vince could go for.
Margaery - Rachel Bilson.
Walder Frey - Charlton Heston, hands down.
Catelyn - Connie Nielsen, as Lucilla in Gladiator, Nielsen never got to really get mean, but I bet she could.
Euron - Robert DeNiro, dark, moody, and he could freak out the freaks.
Eddard - Viggo Mortensen, practically perfect.
Davos - Alan Rickman, I think Rickman could find the soul of the upjumped smuggler and play the loyal follower of the doomed king.
Stannis - Sting, I really think Sting would be believable as the inflexible Stannis.
The Old Bear - Morgan Freeman, sure Westeros is a fantasized England, but Freeman could play the grizzled commander who knows his legacy might not even matter.
I thought of a number of actors, who are too old or who have passed away, who would have been great for these roles.
Drogo - Carter Wong, he played Thunder, the muscular henchman, in Big Trouble in Little China.
Aemon - Marlon Brando, of course I'm imagining an obese Aemon.
Varys - Marlon Brando, again.
Tyrion - David Niven, yes, he's too, too good looking, but that's what make up is for.
Walder Frey - Henry Fonda.
Lyanna - Jaqueline Smith.
Olenna Tyrell - Jessica Tandy.
Eddard - Michael Douglas.
Dolorous Edd - Tim Conway or Bob Newhart, does deadpan get any better than these two?
Margaery - Valerie Bertinelli.
Tywin - Gene Hackman, when he was younger he could have sent chills down your spine as the Lion of Lannister.
Rhaegar - Christopher Reeve, the tragic prince would have been well done in looks and acting by Reeve.
Theon - William Zabka, any fan of The Karate Kid remembers Billy Zabka.
Victarion - Toshiro Mifune, come on, you know Mifune could have been a great viking!
Euron - Oliver Reed, he could brood and be a borderline sociopath very well.
Brienne - Kathy Bates, yes she's too old and too short, but she'd have made a tremendous "stubborn wench."
Arya - Jodie Foster, remember her in Tom Sawyer? She'd have been nails as Arya.