Christoph Waltz fits our image of the world's only friendly giant for many reasons, both real and imagined (i.e. our sneaking suspicion that he would make a very huggable paternal figure, despite never having actually played one). In Inglorious Basterds and Water for Elephants Waltz played dark, unredeemable characters. In Horrible Bosses and Django Unchained, he provided comin relief. The BFG is an inherently good character cut of supposedly evil cloth. He's rough around the edges and unintentionally funny in a style that's part-Hagrid, part-bad-Dad-jokes, and part-Sloth-from-Goonies. The fact that Waltz has yet to work with Spielberg is just icing.
Notwithstanding Helen Mirren's pseudo-monopoly on the role, it's still surprising that Dame Judi Dench has never portrayed the second Queen Elizabeth. Anyone who's seen Dench (Philomena, The Best Exotic Marigold Hotel) as M in the James Bond films can attest to how coldly intimidating, yet alternately warm she can be. That dichotomy, as sovereign as well as mother, wife, and daughter, can't be easy to portray, but it's certainly necessary in playing Queen Elizabeth II.
The Head of the Army is a comically irrational character in The BFG who is obsessed with guns. At first, the HotA is not particularly helpful when the Queen commands him to handle the Giant situation, but HotA and his Air Force counterpart eventually relent. Steve Coogan (I'm Alan Partridge, Ruby Sparks, Philomena) is that rare combination of an incredibly believable character actor and gifted comedian. Read the book again if the symmetry between Coogan and his character is not as obvious to you as it is to us.
Amandla Stenberg got her start as a young Zoe Saldana witnessing tragedy in Colombiana, and in The Hunger Games, as Katniss's young companion Rue. In The BFG, Sophie is young, alone, and way out of her league, though not without a critical and precociously ethical mind. As Rue, Stenberg played an isolated tribute who refused to compromise her integrity under threat of death. That experience will be helpful portraying Sophie's simultaneous innocence and cunning
As Tweedledee to Coogan's Tweedledum, Simon Pegg could be the perfect bumbling, belligerent British Air Force commander. Like the HotA, the BAFC is a role that could be quite dull if not played with the righ comic pizzazz.
It used to be hard to think of Sacha Baron Cohen as anything other than Borat or Bruno, but thanks in part to his role in Les Miserables, he's officially the chameleon of all outlandish characters. In The BFG, the Sultan of Baghdad is Dahl's satirical nod to a postcolonial Iraqi sultan, who, at one point in the novel, alludes to his country chopping off heads like others chop parsley. With Spielberg's artful mastery of adventure film and Mathison's handle on the young adult mainstream audience, it might be possible to make satire clear to theaters, goers and who better to do it than Cohen.