The comic book legend passed away in November (18) at the age of 95, with fans around the world, both famous and non-celebrities, mourning the loss.
As well as his extensive work in comics, including co-creating characters like "Spider-Man", "Thor", "Iron Man", and "Black Panther", Stan had become well known for his cameos in Marvel's superhero movies, and the upcoming Avengers flick will mark his last time doing so.
"It's his last one committed to film," Joe told Mashable. "I have to say, I think it's astonishing that this would be his last cameo. It's just kind of mind-boggling that he made it to the end of this run. I can't believe it."
"Avengers: Endgame", which Joe directed with his brother Anthony Russo, began shooting in August 2017, and finally hits cinemas from 24 April.
Recalling memories of Stan's set visits, self-described "comic book nerd" Anthony, who has directed numerous Marvel films along with brother Joe, revealed how everyone was always in awe of the late star.
"We have all these movie stars on that set, and when Stan came to set, everybody would be like kids again," he said. "It was always a lot of fun shooting these cameos because he was very charming, and he would maybe get the crew going, and everybody would be laughing and having a good time."