Murder mystery movie Knives Out was a smash hit following its release in October as fans were left hooked in trying to solve the intriguing whodunnit.

Daniel Craig leads an all-star cast as detective Benoit Blanc, who is anonymously hired to investigate the mysterious death of Harlan Thrombey - with all of his family members identified as suspects.

A breadcrumb trail of small clues are littered throughout the film to help viewers reach the correct conclusion and navigate the brilliant twists and turns along the way (stop reading if you haven't seen it yet, as this article contains spoilers).

In writer-director Rian Johnson's audio commentary of the film where he explains his decisions in the script and filming process, he reveals a clever clue to the killer's identity many of us missed.

The murder mystery is unravelled by detective Benoit Blanc
In another scene, one of the jugs in Ransom's apartment is shown to be missing

When Harlan's nurse Marta visits his grandson Ransom, Rian explains: "If you take a look in the background, notice there’s a jug missing from the line of jugs on his shelf.

"And that is because he throws one of those jugs through the window in the end sequence to burn down the medical examiner’s office," as reported by Screen Rant.

When the mystery begins to unravel and Ransom is caught out, the director features the full shelf of jugs to expose the earlier clue.

The empty space on the shelf is only briefly shown in the final cut, and Rian explains in the commentary the original shot was planned to be much longer, indicating the jugs were intended to be a more noticeable clue.

The director previously let slip an industry secret which could help you solve murder mysteries in the future - and it's all to do with iPhones.

He revealed: "Apple, they let you use iPhones in movies, but – and this is very pivotal if you’re ever watching a mystery movie – bad guys cannot have iPhones on camera."

Even in Knives Out, Linda (Harlan's eldest daughter) is shown to be holding an iPhone - meaning you can instantly rule her out of the running.