Several possibilities. - Edit 1
Before modification by Werthead at 17/10/2009 07:12:03 PM
How can they have non-speaking extras for certain characters and then replace them with other actors in subsequent episodes? With the Kingsguard knights I expect it'd be hard to notice if they just ride through the screen in armor or so, but if the children are shown at all, it'd get tricky...
One is that they are cast as normal, but simply don't have any lines in the pilot. To be honest, Tommen and Myrcella don't have much to do in the whole series of books, and when they do have some more difficult scenes to do (like Tommen becoming king or Myrcella being injured in Dorne) that'll be several years down the line, when either the actors will have gotten better or recasting will be necessary anyway.
The other is that they are planning to refilm the pilot as Episode 1 when production of the series proper gets underway, and will cast those roles 'properly' then, just using extras for the pilot. This also allows them to recast any roles from the pilot they feel aren't working (ROME and TRUE BLOOD apparently took this approach, whilst DEADWOOD just used the pilot as Episode 1), since the pilot will never be seen by the audience.