Butters... Well I am not really sure how old he is, but being a medical examiner I just imagined he would be at least late 30's if not 40's. I guess they are a little older for the part, Buscemi is 52, Reubens is 57. But I think the fit the character perfectly with the polka playing and the qwerkiness and geekiness.
I definitely cannot see Simon Pegg for this part......Waldo Butters with a British accent.....not feeling that.

Simon Pegg is an actor, and actors often act in different accents. The fact that he is British isn't a disqualifier.

Here's how Butters is described:

from Dead Beat:
Butters was a little guy, maybe five-foot-three in his shoes, maybe 120 pounds soaking wet....He had a shock of wiry black hair that gave him a perpetual look of surprise that stopped just short of being a perpetual look of electrocution.

"Screw up my life?" He stared a me for a second and then said, deadpan, "I'm a five-foot-three, thirty-seven-year-old, single, Jewish medical examiner who needs to pick up his lederhosen from the dry cleaners so that he can play in a one-man polka band at Oktoberfest tomorrow." He pushed up his glasses with his forefinger, folded his arms, and said, "Do your worst."

from White Night:
Butters was an odd little duck. He wasn’t much taller than Murphy, and she probably had more muscle than he did. His shock of black hair resembled nothing so much as an explosion in a steel wool factory. He was all knees and elbows, especially in the surgical greens he was wearing, his face was lean and angular, his nose beaky, and his eyes were bright behind the prescription glasses.

Sorry but 5'10" (approx.) and not-odd-looking-at-all David Wenham would be totally wrong for Butters imo.

Molly... so she looks older than her age, I don't think that means when she is 14 she looks like she is 25, I think it means when she is 14 she looks like she is 17.

But she is around 17-18 in Proven Guilty, when she actually becomes one of the main characters, not 14.

Dakota is a little skinny, but she has played similar roles, Push and Twilight Saga, so I think that qualifies her more than anyone else suggested thus far. Plus yeah she looks young right now because she is 16, she might even fill out some to.

Maybe she could bulk up but can she grow taller than 5'3"?

Description of Molly from Proven Guilty:

She strongly favored her mother, Charity. Both of them were tall for women, only an inch or two under six feet, both of them blond, fair, blue- eyed, and both of them built like the proverbial brick house, somehow managing to combine strength, grace and beauty that showed as much in their bearing, expression, and movement as it did in their appearance.

I just cannot see Dakota Fanning as Molly at all.

But I could definitely get behind Stephen Lang as Morgan though, much more than Jason Statham, who imo, has rather limited acting abilities.

Sorry rose but have to disagree with most of these for a movie .....Meg Ryan (48) as Murphy -  I loved Meg Ryan when I was growing up. Michelle Pfeiffer, Kathy Ireland, etc. These were my generation of celebrity females. But Meg is little old now to be Murphy. And Meg didn't age well either.

I have to disagree with that list too, and you're so right that Meg didn't age well. Not to mention, the bad plastic surgery that turned her into the Joker with fish lips.

Murphy is a hard one to cast, because I have a very specific image of what she should look like from the books:

from Storm Front:
Karrin and I are a study in contrasts. Where I am tall and lean, she's short and stocky. Where I have dark hair and dark eyes, she's got Shirley Temple blond locks and baby blues. Where my features are all lean and angular, with a hawkish nose and a sharp chin, hers are round and smooth, with the kind of cute nose you'd expect on a cheerleader.

from Blood Rites:
Lt. Karrin Murphy, the woman in charge of the Special Investigations division of Chicago PD, stood an even five feet...She was attractive in a pleasantly wholesome kind of way-crystal blue eyes, clear skin, an upturned nose.

Murphy wasn't going to qualify under anyone's definition of willowy or svelte , but she had the build of a gymnast-tough, flexible, and strong.

I can't think of any actress for the role myself, although I can certainly think "no!" to suggestions.

@ MoSeS_

Re: the poll for who could play Harry

Hmm....I disagree with most of those choices - LOL! I love Nathan Fillion but I cannot see him as Harry at all. He'd make a better Michael but I picture Michael as being older.

Murphy - I think Hayden Panettiere's built is what any actress playing Murphy should have, but Hayden Panettiere would be totally wrong for the role. The problem is finding actresses with that sort of built, especially in Hollywood. Even if they start out with a more solid, muscular body type, they almost inevitably diet and exercise to become your typical skinny size zero. Look at the difference between Buffy S1 Sarah Michelle Gellar, and S2+.

I guess for me, I like to see an actor or actress actually look like the character as described, so when I read many of the suggestions, I just go, too thin! Too scrawny! NOT built like a brickhouse! NOT handsome enough! Too porky! I'm just particular that way.  

Boris Kodjoe for Sanya - Check out this video of him. Also, he's going to be in the new J.J. Abrams spy series called Undercovers.

Kincaid - I think ASkars trumps Jason Statham any day! Besides, isn't Kincaid supposed to have long blond hair that he wears in a ponytail sometimes? Would be problematic for Jason Statham unless they put him in a hairpiece.

Ramirez - guess I still have bad associations when it comes to Freddy Rodríguez since I mostly know him from Six Feet Under, and I pretty much hated Rico towards the end of that show! Besides, Gael Garcia Bernal would make a very pretty Warden. Albeit, a tad hard to believe as a virgin.....

Bill Nighy for Nic - as much as I am a stickler for "actor must look like character as described in the books," I'll make an exception here as I just love Bill Nighy and he would be awesome as Nic.

Megan Fox as Madeline Raith - I chose Megan as I actually do not care for her. She's been described as the poor man's Angelina Jolie, and to me, she is perfect as the overblown, too-obvious, trying-way-too-hard Madeline. Somehow, I always pictured her as being younger than Lara.

Vincent Cassal as Lord Raith: I agree he has the predatory cruelty quality, but handsomer than Thomas? Any of the Thomases suggested so far? That's a major fail for me.

And I thought of someone for Butters: Simon Pegg! Here he is, with dark hair.