There's one major clue in the opening moments of Shutter Island that viewers missed first time and gives away ending

Watching Shutter Island for the second time is a completely different experience when you know the shock ending.

The Martin Scorsese film stunned audiences in 2010 after the massive twist was revealed at the end as we follow US Marshal Teddy Daniels as he attempts to solve the disappearance of a patient at Ashecliffe Hospital on Shutter Island.

It is then revealed at the end that Daniels (Leonardo DiCaprio) actually Andrew Laeddis, himself a patient at the hospital.

Daniels was part of an investigation involving role-play therapy, organised by Dr. Cawley (Ben Kingsley) to help him confront his past.

I mean, who can forget the final scene?

When Teddy asked his partner Chuck (Mark Ruffalo): “Which would be worse, to live as a monster or to die as a good man?” making it hard to know if he knew what was happening all along.

But, according to a film expert at Looper, the clues were always there for the audience to figure out that something was up.

Here's the opening scene, so see if you can spot the clues?

What are the clues?

In one of the opening scenes, Teddy doesn't have a cigarette lighter, and as a patient, he's probably not allowed to have one because it's a dangerous item.

"Additionally, the boat's bathroom area looks suspiciously like the kind of cell where a patient living in a 1950s institution would be held," Looper added. "But it's the meeting of Chuck and Teddy, that should immediately tip off viewers that something strange is afoot."

They also point out the moment when Teddy and Chuck come face-to-face with the facilities' guards, who appear to 'tense up and grip their rifles a little tighter'.

While the initial assumption might be that they are hiding something, Looper goes on to say that the guards are 'nervous' as they don't know if Teddy will 'snap at any second'.

The ending was quite something (Paramount Pictures)

Now, another big clue you might have missed is when the US marshals are asked to give up their guns.

"There's a prolonged moment where Chuck fumbles getting his firearm out of its holster," Looper explained. "If he were actually an experienced Marshal, he should have no problem getting it undone.

"The reason he has an issue is that he's really a doctor and he doesn't know how to handle a gun properly.

"At first, the scene comes across as amusing, but it gives up the entire twist right there."

So there you have it, Shutter Island is one of those films that, the more you watch it, the more you'll find out, which is always nice when revisiting a movie.