People point out the key difference as Tom Cruise and Will Smith both pose on top of Burj Khalifa

When Tom Cruise and Will Smith each had their pictures taken on top of the tallest building in the world, people quickly noticed one key difference between the two Hollywood stars.

The Burj Khalifa, literally 'Khalifa Tower' in Arabic, is in Dubai in the United Arab Emirates, and holds the record for the tallest building in the world.

Named after former UAE president Khalifa bin Zayed Al Nahyan, it towers some 829.8 metres above the city of Dubai.

Both Tom Cruise and Will Smith managed to get all the way to the top of the immense structure. And we're not just talking the top floor here either, they went to the top of the spire.

The highest elevator access in the building goes to the 160th floor, with floors 155 and above being reserved for maintenance telecoms companies.

There are offices up to floor 154 and the general public can visit up to floor 148.

Visitors are only allowed above that with special permission, for example, if you're a multi-millionaire celebrity like Cruise or Smith.

And people noticed one key difference between the two stars' photos up there.

Will Smith on top of the Burj Khalifa, looking like he's slightly losing his mind. (YouTube)

Sitting on a small spire more than 800 metres above the ground would be a nerve-wracking experience for most people.

And here 'most people' definitely includes Smith, whose smile for his photo looks more like a grimace.

The Men in Black star climbed the tower as part of a video on fitness and weight loss. At the top, he appears to be trying to maintain his composure while gripping onto two boxes either side of him.

For Cruise however, it's a very, very different story.

The Mission Impossible star - who's credited for doing his own wild stunts in his films - climbed the tower for a stunt in Mission Impossible: Ghost Protocol.

The photo shows him perched on top of the boxes wearing a casual outfit, and barefoot.

Not only that, but he was leaning over to one side as though he was casually chilling on the beach.

Tom Cruise on top of the Burj Khalifa, looking incredibly unphased. (X)

While he doesn't appear to be wearing a harness, fact check websites have suggested that he almost certainly is.

People took to social media to share their thoughts on the comparison, with one writing: "One is sitting in bare feet and a t-shirt looking as relaxed as possible and the other is draped in harnesses, clinging on for dear life, looking like he's making peace with God."

A second posted: "I love how Will is all strapped up and hanging on for dear life, but Tom is not only just sitting there, nary a rope in sight, but he’s LEANING."

And a third commented: "Tom looks like he is posted on his front stoop just vibing and Will is the living embodiment of terror.

"That’s not a smile that’s a death grin. He’s got 63 harnesses tied to him. Legend says he’s still holding those boxes."