Different Actors IN different Episodes

[9inchsword]

Hi, has anyone ever noticed that in series 3 (as well as other series) the characters present between each episode would rotate, and not as such change from quest to quest

For example Mellisandre was present in the epsidode with both the end of Team 8 (died through a bomb) and the start of team 9 (she waved them on for some pointless reason), and yet was not present in the immediate episode after with the same team.

Therefore i jusify that the actors worked on a pro-episode shift, hence going through the actors until it reached their turn again