Just an excerpt from the article from April 25. Article may contain spoilers by some standards:
Tustin: Hey there, Mr. Cusick...how much are you (and other actors) allowed/encouraged to ad-lib or take liberties (however small) with the scripts? Thanks.
Henry Ian Cusick: Very little. So many clues in the lines, so it's pretty tight.
It is quite long, so to read further, clickie the link.