It's classic contractual obligation performance syndrome.

They turned up, stood on the spot directed and said the lines.

In the other two films they ARE the characters in this they are the actors playing at playing the characters (sort of like Mark in The Muppet Show only less entertaining).

With Carrie it's more not having much to do.

In the other films she is an authority figure with man trouble.

In this one she is demoted and the trouble fixed itself with no necessary effort from herself.

Harrison doesn't seem to give a damn about Han anymore (that terrible face pulling interlude on Endor is painful to watch, everything else plays like he has is half asleep or drunk or both).