Williams was sworn to secrecy about the part she was playing and had to sign non-disclosure agreements, though ultimately news of her casting as young Cersei leaked out on IMDB months before her scene aired in the Season 5 premiere, "The Wars to Come".
Williams gave an interview with MTV.com after the Season 5 premiere, giving details about playing young Cersei. Due to her age, Williams had never seen the TV show, nor read any of the novels. In the few weeks after arriving in Northern Ireland but before filming began, she tried starting the novels but to no avail - their length relative to the specific scene she needed to film was too disproportionate. Instead, she focused on closely studying all of Lena Headey's scenes from the first four seasons of the TV series, and closely studying her mannerisms. As Williams said:
- "I got the accent quite quickly, but what was quite difficult was finding out what made her. She's obviously very nasty; extremely nasty. Because I was playing her when she was younger - she couldn't always be horrible. She could be a bit stuck up when she's younger, but I think the importance of this scene is making sure the audience knows how she became the person she is. And that was really interesting to do. So I just had to find out as much as I possibly could about her."
Williams also explained that even though her scene was only a little over four minutes long, it took about two weeks of on and off filming to produce. Filming on Game of Thrones is more like filming a movie, with hours of extensive preparation to arrange the set, position the lighting, etc., so they only actually filmed a minute or so at a time each day (and filmed multiple takes of the same minute).