Nicole Kidman plays Grace Fraser in The Undoing, which has received rave reviews since it aired in October.
Fans won't have long to wait to find out who killed Elena in the HBO drama, with the final episode airing on Sunday night in the United States, and Monday evening in the UK.
Fans have been guessing who the culprit could be throughout the series, and were left in shock following the last episode's ending, which saw Grace discover the murder weapon inside her son Henry's violin case.
VIDEO: The Undoing starring Nicole Kidman and Hugh Grant
Nicole has been enjoying reading all the different fan theories, which have listed everyone from Grace to her father Franklin as the killer, and recently responded to a post from Katie Couric detailing her thoughts of the potential ending.
Nicole Kidman plays Grace Fraser in the HBO drama
Katie had written: "Okay I am full out obsessed with @undoinghbo and have several theories. SPOILER ALERT! If you haven’t watched the first four (soon to be five) episodes DO NOT READ ANY FURTHER!!!
"Okay, fire away everyone! whodoneit UPDATE:!!!!! Send me your new theories!!!!!
Fans won't have long to wait to find out who killed Elena
"PS. Are there hints in the open (one of my favorite songs...love the Mama Cash version...that my new bestie (Nicole) actually sang! (Beautifully of course) Pocking the daffodils, the flash of blood, the bubble bursts, innocence lost. Hmmmmmmm..."
Nicole stars alongside Hugh Grant
Nicole replied, writing: "Katie, my new bestie! Thank you so much for supporting #TheUndoing. I absolutely love the details you picked up on, and love that the opening is one of your favorite songs xx."
Other followers replied with their own theories about the suspect. One wrote: "Her dad hired a hitman and framed Jonathan," while another wrote: "Their son! And Grandpa isn't innocent either. Also the blonde friend, something up with her too!"
Nicole's dad is another suspect
A third added: "I think she did it! Why would she be recalling these past events?" A fourth wrote: "Could she have multiple personality disorder? I feel like it's her dad. But man, she's going in and out of memories a lot."
Nicole and Hugh Grant, who plays her husband Jonathan in the show, had the best time working alongside each other, having been friends for many years.
Henry's violin case was holding the murder weapon
Chatting to Marie Claire with Hugh, the mother-of-four said: "Well, I like you. So that was a really easy part… Because there was something very relaxed, obviously, because I'm Australian and you're British, we have that… I don’t know, it’s just a sense of humour. It’s good chemistry."
Was it Grace Fraser who killed Elena?
Hugh agreed and Nicole continued: "We talked... I trust you. But that's also part of working together, I think, when you really click, then you sort of sit and you do talk and you share things and you open up. You're incredibly honest. I'm always like, ‘Hugh, shush, you can't say that.'"
Read more HELLO! US stories here
Like this story? Sign up to our newsletter to get other stories like this delivered straight to your inbox.