Rie Tanaka confirmed at her appearance at the 2008 NYAF that Lacus was the hardest role she played so far:
GM: What’s the most difficult role that you’ve played, and how did you overcome it?
RT: [Gundam SEED's] Lacus Clyne, because the character has to be very reserved and commanding, even if she’s mad or showing some expressions, she still has to be a reserved character, and this was over a span of four years including Destiny, and it was very difficult.
http://www.animevice.com/news/nyaf08...rie-tanaka/78/