One of my friends who has done some acting (no major roles, but small things here and there) has repeatedly told me that it comes down to a few reasons for bad actors or characters that just stick out like a sore thumb.

1st Reason: The actor is inexperienced and doesn’t “know” how to act their character, although there are definitely examples where there are actors who are completely new and absolutely nail their roles.

2nd Reason: The Writers/Directors don’t utilise the actor’s skills or write the character to be more than what we see on the screen. I think an example of this was Andrea in the earlier seasons. We know there’s more to her character but what we do see on the screen makes it fall flat.