Avengers: Endgame was a who's who of the Marvel Cinematic Universe. In addition to all the major heroes that we saw, the movie brought back a crazy number of important characters, many of whom had previously died, thanks to the movie's time travel focus. We saw the return of Tilda Swinton, Rene Russo, Robert Redford, and more.
With such a star studded list of cameos, it wouldn't be a shock if some of them were difficult to make happen, but director Anthony Russo says the most difficult "cameo" in the movie to schedule, actually involved three members of his extended family. According to Russo...
The hardest one was a scene in the movie where it’s the scene in the diner where we introduced Smart Hulk the first time. So, he’s sitting there with Ant-Man, Black Widow and Captain America, and there’s these three kids that walk in who are fans of Hulk. They want a picture with him. And those three kids are, one is my son Julian. One is my brother’s daughter, Lia. One is my sister’s son, Augie.
Anthony Russo tells Cleveland.com that, because of school schedules, getting the three kids together to film the scene with the Avengers: Endgame stars was actually the hardest cameo to make happen.
These three aren't the only members of the Russo clan to appear in the film. Joe Russo's daughter Ava portrays Lila Barton in the film's opening scene. She's the daughter of Jeremy Renner's Hawkeye who is learning archery before becoming a victim of Thanos. Her sister is Lia, one of the kids in the scene with Hulk.
The other reason the scene may have been somewhat difficult is that it was an entirely new scene added as part of the reshoots on Avengers: Endgame. The Russo brothers explain in the commentary track on the new Avengers: Endgame Blu-ray that, originally, the fusion of Bruce Banner and Hulk was meant to happen at the end of Avengers: Infinity War. However, it was decided in editing that the scene didn't work with that movie's otherwise depressing finish.
The filmmakers then had to scramble to add a scene in Endgame to explain the change in Hulk. Reshoots are a standard part of filming these days, especially with big movies like Avengers: Endgame, so getting the stars like Chris Evans and Paul Rudd back was likely no problem, they were already planning to be there, but getting the non-stars together to complete the scene was something else.
The easiest cameo to make happen was probably Natalie Portman. Her scene was done using old footage from Thor: The Dark World, though she did do some additional voice work for Endgame.
Apparently getting Tilda Swinton to look like the Ancient One again, or having Robert Redford cameo in what may end up being his final on screen role, was easy compared to making sure three kids didn't miss any school. It makes it sound like filming Avengers: Infinity War and Endgame back-to-back was as simple chore, which we very much know not to be the case.