Avengers: Infinity War is the culmination of every moment in the Marvel Cinematic Universe in one big team-up.
Speaking with Comic Book, co-director Joe Russo was asked if the Guardians would show up and replied:
“That is another one that we would have to reserve as a surprise, but we will say this: People will not be disappointed in the amount of characters in the movie. The concept of Infinity War is that the Marvel universe unites to battle the greatest threat to the world and universe that you’ve ever seen, and we’re going to honor that concept.”
Russo went on to talk about the film’s cosmic nature and how it would be different from the grounded approach he and his brother Anthony brought to Captain America: The Winter Soldier, and presumably Captain America: Civil War.
“I sat in the theater and watched Empire Strikes Back seven times in a row when I was a kid. Yeah, we’re science fiction fans as well, Dungeons and Dragons. We collected all the books when we were kids. We had figurines. Anything that you could think of that has to do with fantasy or sci-fi or superheroes was in our universe. We’re just going to approach the cosmic side with the same fervor that we did the earthbound work we’ve been doing. We also are going to work really hard to bring a psychological realism. We obviously can’t bring a naturalism to it because it doesn’t really exist outside of science fiction, but we can bring that psychological realism to it, to the characters, and that’s going to be our focus… Also, it’s going to be our focus to explore the cosmic side, applying the same rules that we like to apply to the last two films that we’ve done. It ends up being the rules of physics always apply or apply as much as we can get them to apply based on what we’re doing.”