EXCLUSIVE: 'Game of Thrones' Actress Keisha Castle-Hughes Geeked Out on Set Over Jaime Lannister!

This video is unavailable because we were unable to load a message from our sponsors.

If you are using ad-blocking software, please disable it and reload the page.

Warning: Spoilers from previous episodes of ‘Game of Thrones’ lie within this post. If you’re not caught up, come back later!

Oscar-nominated actress
Keisha Castle-Hughesis the spear-trained, lethal assassin Obara Sand – eldest of Prince Oberyn Martell aka the Red Viper’s eight bastard children -- on HBO’s Game of Thrones.

In a new sit-down interview, Castle-Hughes tells ET about the night she found out she was cast -- casually, the same night her TV father would meet his end at the forceful hands of Ser Gregor Clegane (dubbed ‘The Mountain’ due to his size) in the season four episode, The Mountain and The Viper.

WATCH: 'Game of Thrones' Season 6 Finale Killed Pretty Much Everyone and Revealed a Game-Changing Twist -- Relive the 6 Most Insane Moments!

“I don't know if that was planned? It was really strange,” questioned Castle-Hughes, who watched the episode at her home that evening. “I had such an emotional reaction to it!"

After learning of her casting, Castle-Hughes said she felt connected to Oberyn (Pedro Pascal) – well, at least for the few hours of TV-life he had left… before his skull was violently crushed by The Mountain.

“I was crying so hysterically and [my roommate] came in and she's like, 'Dude, it's just a TV show! You should know that. This is what you do for a living,’” recalled Castle-Hughes. “Obviously at that point, I'd signed one million non-disclosure agreements with HBO and so I couldn't tell anyone anything! I was going, 'You don't understand why this is so important!'”

EXCLUSIVE: Keisha Castle-Hughes Finds Success on TV With Cameron Crowe's 'Roadies' and 'Game of Thrones'

Castle-Hughes was a massive fan of George R. R. Martin's book series, A Song of Ice and Fire, before she was cast, so it was understandably difficult not to geek out over certain props, sets, and characters in the flesh.

Jessica Henwick, who plays Nymeria Sand – I remember the first day we got to set -- the two of us were holding onto each other shaking, because we were so nervous and so excited,” gushed Castle-Hughes, whose early scenes with the eldest Sand Snakes included confrontations with Jaime Lannister (Nikolaj Coster-Waldeau) and Bronn (Jerome Flynn).

“Both of us were completely speechless. [Nikolaj and Jerome] came over and said, ‘Oh, hi! Welcome. Nice to meet you. Welcome,' and both of us were like, 'Oh... yeah.... you're cool!'” Castle-Hughes laughed, “It took a while for that to dissipate.”