Luther - Paradise Circus

Theme song from Luther by Massive Attack - Paradise Circus - Video by Valentine Petishkin
We've been watching Luther, the BBC copper drama starring Idris Elba. For Series 4 they seem to have upped the depravity, violence and inter-office strife. It's still a great show, Idris is the understated strong man. We've also got Sienna Guillory as Luther's new girlfriend Mary Day, and she is certainly good looking.
   Got to wondering this evening whether stars, like Idris, have real "star power" or whether the mannerisms of the star are scripted right down to the twitch of an eyelid. I mean, say you've got six characters in your show, and you've that many actors to play their roles. How do you decide who plays which role? Do you go by gut instinct, or feel? Or do you just randomly assign people to roles, and if they are good actors or actresses they simply play the role they have been assigned? Do we even call female actors actresses anymore, or is everyone an actor?