Basically, whether or not the actor had enough lines and material this season for me to get a bearing on their work. In some exceptional cases, I might mention actors that pretty much didn't get anything, but still did a very good job with what little they had and deserve a kudo. Some examples of actors that won't appear here, for want of material, would be Princess Myrcella, Rodrick Cassel or Grenn/Dolorous Edd. They were there, but they didn't really do anything, save for like Rodrick whose job was to foam at Theon for one scene (which he did wonderfully, but it's hard to compare him to others).
--> Everyone on this list has been featured in at least two scenes.
--> Aside from Matthos, the 'oddballs', I'd say, would be #16, 18, 21 and 24. The rest have been featured at length.
I'll probably try to do some more for today.
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When you have nothing to say, quote yourself ~ Solfadore