To begin with some minor characters...
Charles Dance is simply the scariest man on earth. He's got a splendid air of menace and grandeur. And since Darnay's grand bad uncle is there to be grandly bad and nothing more, he's perfect. On the other hand, this part can be quite fun when someone turns it campy (see Christopher Lee), and Dance has a wonderful dry sense of humor, on display in the excellent Terry Pratchett movie, Going Postal.
But who could play him in flashbacks?
Young Marquis St. Evremonde
Jarvis Lorry - Bill PatersonAfter Jerry Cruncher comes his boss, the fussy, no-nonsense, heart-of-gold banker Jarvis Lorry. Bill Paterson is a hardened period drama veteran - in this photograph playing Mr. Meagles in Little Dorrit. The real question is whether he can handle both the fussiness of the character and yet give him a bit of depth (I think he can), for that scene near the end. You know, with the fire and the log and Sydney's boot...you don't? You're not as obsessed with this story as I am? You've got problems.
oblivious idiot like no one else. Here he is in Cranford.
True Grit, so I know she has the acting chops to handle a brief but pivotal role. (I'd also be happy with Q'orianka Kilcher.)
Gina Dalfonzo and Rachel McMillan. John Hurt has just the right qualities of dignity and quietness to play Dr. Manette. Also, importantly, I think he could do crazy.
And since, given the flashbacks, we'll also need a younger Manette, how 's about...
Inspector George Gently series, but he's also terrific and nearly unrecognizable as the under-confident pushover Midshipman Hollom in Master and Commander: The Far Side of the World.
And our finalists?
Lucie Manette - Felicity JonesLucie Manette will be played by Felicity Jones. She's had plenty of period drama experience from The Invisible Woman to Northanger Abbey. Recently she was in The Theory of Everything, playing Steven Hawking's wife, Jane.
Charles Darnay and Sydney Carton - Karl Urban and Dominic Cooper
And...Darnay will be Karl Urban. No period drama experience, as far as I can tell, but he's great with accents, and if he's got the dignity to play Eomer, he can handle Darnay.
Agent Carter as very American Howard Stark, but he is, in fact, British, as one can see in the below screenshot from Sense and Sensibility.
And that's it. A Tale of Two Cities. Not the dream cast, the only cast.