The Daily Record has a very spoilery article on the final two episodes of Little Dorrit. You can read it here. (thanks Steph!)
It had this to say about Matthew Macfadyen:
"... and Matthew Macfadyen as Arthur Clennam, the rock around which all previous storms have dashed themselves.
Macfadyen's role could easily have been a bore but he somehow managed to convey Arthur's saintly, solid reliability without ever making it tedious.
That solidity is shaken in the final episode..."