Warning: This article contains SPOILERS for Avengers: Endgame.

The cast of Avengers: Endgame celebrates Iron Man's 11th birthday on the set of the Joe and Anthony Russo-directed film. Helmed by Jon Favreau and starring Robert Downey Jr. as the headlining character, much has been said about how Iron Man birthed the MCU. Knowing just how big of a risk Marvel Studios president Kevin Feige and his team had to take to get that project off the ground makes one more appreciative of it and the universe it kicked off. And there was no better way to pay homage to it than what the Russos did in the Avengers: Infinity War sequel.

Dubbed the culmination of everything that has transpired in the franchise in the last decade, Endgame also brings closure to the personal journeys of the MCU's beloved heroes, including its inaugural, Tony Stark. But before Iron Man's sacrificial death to defeat Thanos in the film, Marvel Studios brought together almost every major character in the universe in an epic fight scene against Thanos and his minions. Fans have already seen behind-the-scene snippets of what it was like working on this massive action set piece through social media postings from Chris Pratt (Star-Lord) and Elizabeth Olsen (Scarlet Witch), but the public gets a new look at it thanks to a new upload from Downey.

Related: Every Character At Tony Stark's Funeral In Avengers: Endgame

Taking it to his official Instagram account, the actor shared a short video of him alongside several of his Endgame co-stars enthusiastically singing "Happy Birthday" in celebration of Iron Man's 11th year from its theatrical release. In the clip there are Pratt, Chris Evans, Mark Ruffalo (Hulk), Chris Hemsworth (Thor), Evangeline Lily (Wasp), Danai Gurira (Okoye), Tom Holland (Spider-Man), Chadwick Boseman (Black Panther), Paul Rudd (Ant-Man), Pom Klementieff (Mantis), Sean Gunn (mo-cap for Rocket), Olsen, Antony Mackie (Falcon) with director Joe Russo on the side and Downey in the middle. Watch the clip below:

Unlike previously released BTS videos from the set of Endgame, this one shows a majority of the heroes in their full costumes - except for Holland and Ruffalo who are both wearing their mo-cap suits. There's something unsettling seeing a  battered Thor, as well as a bloodied Captain America and Iron Man dancing and singing goofily knowing what their fates are in the film. From the looks of it, it seems like they were filming parts of the big fight sequence towards its end, hence the beaten up faces of the heroes. And for them to actually break out of character to record this video is quite fascinating.

The miracle that is Iron Man, reinforced by the idea that it was the foundation of the MCU, has been discussed multiple times in the past. It set the feel and vibe of the whole universe, and Tony Stark has become the archetype of MCU characters - flawed beings who just want to do what's right. But seeing these many stars seemingly enjoy themselves genuinely is a testament to the working conditions in these superhero movies, and a huge part of that is Downey making it feel like everyone's welcome. Avengers: Endgame is a love letter to Favreu's film with numerous callbacks to the 2008 flick, all in service of the satisfying ending of Stark's journey.

More: Avengers: Endgame's Post-Credits Surprise Explained

Source: Robert Downey Jr.

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