See, part of me agrees with the unknown actors bit, at least for the lead roles. Part of me thinks that lesser known actors that are still good would be best, so that when people watch, they see the characters, not the actors they know playing the characters.
On the other hand, putting in some well known actors would bring some legitimacy to the show, and garner fan interest. For instance, casting Nathan Fillion as Michael Carpenter would interest his entire fanbase, and might increase viewership. But it has to be the right fit for the character first.
I think having guest appearances by big names would be best. I like the idea of Helen Mirren as Mother Summer and Judi Dench as Mother Winter. Big names that can also act the part convincingly. And I like the idea of casting capable actors in really small parts too. I've always pictured Michael Chiklis as Mac, even though he doesn't fit the physical description. But the first time I thought of Michael Chiklis in the series was page one of SF, when JB described the new mailman as looking like a basketball. I immediately though of Chiklis. Casting more recognized people for smaller parts would definitely be a plus, as long as they fit.
I can easily concede you those points.
Lesser known actors, such as Peter Dinklage who has done a fair bit of filmwork prior to starring in Game of Thrones, pass off their parts with aplomb and weight that most of us simply wouldn't have expected. Despite familiarity with the actor and a great deal of character discrepancies (Tyrion's appearance vs Peter's), it just
feels right that he was cast for that part.
I'm a huge fan of Nathan Fillion (admittedly mostly from his work on Firefly and thereafter), and despite those very impressionable roles, he still has some flexibility and hasn't been typecast in his works as strongly as, say, Timothy Olyphant (Deadwood, Justified; always the dignified but loose cannon pretty-boy lawman). He could certainly do, as you exampled, Michael convincingly. But could someone do better? I think so. And I'd take that risk in a heartbeat, despite my manly love for Nathan.
There's also the concern that well established actors justifiably get paid more, and that would detract from the film's overall budget. Certainly the film could gross more due to the fanbase the actors bring with them, but it doesn't change the budget of the film and that's very important. Sets & effects vs actor appeal isn't a hard decision in my books. I'm as much a proponent of actor-driven dramas, but the stage has to be convincing too otherwise it all falls apart.
I've been told I'm a pragmatic sort of thinker in this regard, as I remove possibilities based on the above. I still enjoy visualising what people are suggesting for parts though. Without scrolling back over 337 pages, what are the fan favourites at the moment? The poll doesn't seem very indicative.
Poll