Benedict Cumberbatch revealed that none of the cast members were allowed to read the entire script so as to avoid them accidentally releasing spoilers like Tom Holland and Mark Ruffalo often do. Cumberbatch himself though was one of the few allowed to read the whole script. Holland himself was kept in the dark about who his opponent was during a fight scene.
A Malaysian student made a low-budget parody of the first trailer and it caught the attention of the Russo brothers themselves, who jokingly offered him to direct Avengers 5. It gets even better when he and his friends got to meet Joe Russo in person.
Some people believe Robert Downey, Jr.'s line "Do you concur?" is an allusion to the fact that both he and Benedict Cumberbatch have played Sherlock Holmes, and that it is a stand-in for the "No Shit, Sherlock" line that the fans wanted and the filmmakers decided not to use. However, the line is never spoken in either RDJ's Sherlock Holmes (2009) or Cumberbatch's Sherlock. It is a commonly used phrase in medical dramas, though, which is likely alluding to the fact that Doctor Strange is still a medical doctor.
Overlapping with Never Trust a Trailer, Thanos's speech about "[knowing] what it's like to lose" is quoted often, except it's the trailer version being quoted instead of the slightly different one in the final cut. Similarly, the just-as-quoted "This does put a smile on my face" only appears in the teaser trailer.
Covers Always Lie: Hawkeye appears in the 2017 San Diego Comic Con poster for the film, yet he doesn't appear in the film at all.
Creator Backlash: Minor example, but Dave Bautista tweeted in August 2019 that he's quite tired of people making "invisible Drax" jokes on social media, going so far as to admit that he now misses the "sensitive nipples" jokes from Guardians of the Galaxy Vol. 2.
Zoe Saldana indicated in an interview that the second installment would be titled Avengers: Infinity Gauntlet. James Gunn, an executive producer on the film, chimed in and indicated that Zoe was misinformed, and that Infinity Gauntlet is not the real title.
In The first Guardians of the Galaxy, Gamora's rap sheet stated that she was the sole survivor of her species, yet in this film, only half of her people are slaughtered by Thanos' forces, and the latter states the planet is thriving. When asked if this was a retcon, the Russo brothers declined to give a proper answer, only stating that either Gamora or Thanos is lying.
Thor is shown with both eyes intact in the D23 trailer, despite having his right eye brutally ripped out by Hela in Thor: Ragnarok. As his eyepatch was an entirely CGI creation using four dots around his eye the D23 footage had the dots removed to hide the twist in Ragnarok.
Gems were edited out of footage of the Infinity Gauntlet in order to prevent people from guessing exactly at what point in the movie those scenes happened.
A tweet from a Brazilian convention claiming that Feige told the present audience that Loki will be Thanos's ally during Infinity War caused quite a stir on social media. Actually though, Feige only said that they "might show up [in Thanos's company] for a while", leaving his true alliance still ambiguous. In the film, Loki would actually initially seem to want to ally with Thanos, only to betray him, fail and die.
Similarly, some have claimed that Feige or another Marvel bigwig had said long ago that the TV characters would appear and that the fact that they aren't is further proof of a rift between the Marvel Studios film division (which is mostly independent from the rest of Marvel) and TV division (which is part of the overall Marvel Entertainment division owned and run by reclusive cheapskate billionaire Ike Perlmutter). In reality, nothing other than vague claims that eventually TV characters would show up in the movies ever happened — people just assumed it would be IW.
Hostility on the Set: An extremely mild case, and a rather humorous one at that — Benedict Cumberbatch initially ignored Tom Holland, thinking he was just an overeager crew member and fan... and then realized he's a co-star. Some find this extremely bizarre, as they had filmed The Current War together a year prior.
A lot of people called that Loki's Scepter was one of the Infinity Stones based off its appearance in the Infinity War teaser. They were proven right by the object's presence in the teaser, and later in a scene in Avengers: Age of Ultron that revealed this in-canon.
Ditto with the Eye of Agamotto being an Infinity Stone, as revealed at the climax of Doctor Strange (2016).
Fan speculation that the Black Order would be involved dated at least back to an interview in 2015, where Joss Whedon stated that Thanos likely wouldn't be the only villain. They were finally proven right in 2017 when set photos of a stuntman portraying Corvus Glaive surfaced.
When Mjölnir was destroyed in the trailer of Thor: Ragnarok, many fans speculated he would have a new weapon crafted in this movie. Which weapon he would receive was up for debate, but some speculated he would get some version of Stormbreaker. Sure enough...
More than a few viewers had already guessed that the movie would end with Thanos performing his iconic Badass Fingersnap.
There was some speculation that Red Skull survived his fight with Captain America in Captain America: The First Avenger. Sure enough, he appears as the keeper of the Soul Stone in this film.
Vision is the most popular candidate to die in fan theories due to possessing the Mind Stone. Sure enough...
Many also correctly guessed that Loki would die in the first minutes of the movie. He doesn't have a character poster of his own whereas secondary characters like Wong and Okoye have their own, which makes it unusual considering the character's popularity. Not only that, every footage of him in trailers was only from one scene and Word of God previously teased that Infinity War would pick up right where Ragnarok ended.
Many fans correctly realized that the reason Spider-Man wasn't feeling well before he disintegrated was because his Spider-Sense was warning him of his imminent death.
Peter Dinklage, who has dwarfism, plays the largest character in the entire movie.
The naturally blonde Scarlett Johansson plays a red-haired character who dyed her hair blonde to survive being a fugitive.
Jossed: Early on, there was speculation that the Vision would be a Composite Character of Adam Warlock and that the character would not appear in the MCU. However, it would later be confirmed that Adam Warlock would be appearing in Guardians of the Galaxy Vol. 3 shortly after that film was officially announced by James Gunn, throwing this line of speculation out the window. That being said, there's nothing to suggest that Vision won't still take Adam Warlock's role in the story, especially as Adam Warlock was only hinted at in the stinger to Guardians of the Galaxy Vol. 2..
Meaningful Release Date: The film was set for release on May 4, 2018 in the United States, which is the same day on which the first Avengers movie was released in 2012. It was later moved to April 27th worldwide. While the anniversary date is lost, this makes it so The Avengers and Avengers: Age of Ultron were released 1092 days apart, as were Age of Ultron and Infinity War.
Meme Acknowledgment: Benedict Cumberbatch is well aware of how many people really want Doctor Strange and Tony Stark's interactions to include "No shit, Sherlock," but politely declined due to thinking it's a little too cheesy.
Kevin Feige has pointed out that the films can be considered this, since they're the culmination of the past ten years of stories (as an example of this, the Marvel Studios logo at the beginning of this film shows a "10" in place of the "io" in "Studios"). Infinity War was originally set to be released on May 4, 2018, ten years and two days after Iron Man (before getting bumped up a week, still making it "merely" 9 years and 360 days), the movie that started it all. In addition, its sequel was released in 2019 — the 80th anniversary of the Marvel Comics Universe. The "io" of the "Marvel Studios" logo even turned into "10" for this movie, since it came out the year of the MCU's 10th anniversary.
Thanos says that "Fun isn't something one considers when balancing the universe. But this... does put a smile on my face." This is likely a retort to Ebony Maw's "Let him have his fun," during the Hulk fight.
The scene of Steve and T'Challa leading a heroic charge comprised of Bucky, the Hulk, Black Widow, War Machine, and the Wakandan army through the forests of Wakanda. In fact, Bruce spends said battle in the Hulkbuster armor since the Hulk persona refuses to emerge.
T'Challa's "Today, we do not fight for one life. We fight for all of them!" speech.
Thor using his lightning powers at the forge without any weapon and Rocket and Groot staring in awe.
Black Widow, Bruce Banner, and War Machine looking up at the sky in horror.
The Other Darrin: And an incredibly subtle one. The Red Skull, previously portrayed by Hugo Weaving, is instead portrayed by Ross Marquand in his one scene. Marquand is skilled at impressions, and the facial prosthesis makes it additionally difficult to notice this.
Tom Holland: I remember actually going to see the first Avengers with one of my best mates from home, and I'd never have dreamed that I'd ever be in one of these movies, let alone playing Spider-Man. I feel like I'm on top of the world, because this is just mind-blowing.
Reality Subtext: After using the Time Stone to peer into over 14 million timelines, Doctor Strange is the only character that knew for certain whether the heroes would be able to stop Thanos. Fittingly, Benedict Cumberbatch was one of the few cast members allowed to read the entire script.
Release Date Change: The film was originally going to be released on May 4th in the US before Marvel moved it one week earlier to April 27th, so that the American release would coincide with the most of the foreign markets (who usually get MCU films a week before); that way, the biggest market could see the film before spoilers could become prevalent online.
The Russos refused to confirm on the ultimate fates of Shuri, Ned, Jane Foster, Miek and Korg, stating it's a spoiler.
As of Endgame, it's been shown that Ned was dusted and subsequently revived, as was Shuri; Jane is MIA except for when Thor and Rocket go to the past, so there's not confirmation one way or another; and Miek and Korg survived.
The fact that Thor would be getting Stormbreaker as his new weapon was spoiled by the toys for the film months in advance. So was Bruce piloting the Hulkbuster.
A prequel novel called Thanos: Titan Consumed was announced as an official tie-in to the film. After the book's blurb went up online, Marvel issued a response claiming the book was non-canon, leading to several contradictory statements about its place in the MCU canon. It's very likely that Marvel claimed the novel was non-canon when they realized the blurb spoiled that Infinity War would end with Thanos winning.
Teasing Creator: Michael Rooker walked onto the set with a hat lazily covering up the Avengers logo, but he denied to confirm his involvement with the movie. It would later be confirmed that he was not involved with the film. The reason for this was because Yondu died in Guardians of the Galaxy Vol. 2, and this was done deliberately to throw off leaks that his character would die in that movie.
According to Joe Russo, Tom Holland improvised the lines just before Spider-Man disintegrates. Tom later elaborated on this, stating that the scene was rewritten after he did an improvised version, and the rewrite is what's seen in the film. He also specified that the reason he repeated "I don't want to go." was because repeating himself is his preferred way of getting emotional in short amount of time.
Originally, Drax wasn't supposed to reply to Tony Stark's and Peter Quill's respective questions about Gamora. Dave Bautista felt it was appropriate to ad-lib the reply "I'll do you one better: why is Gamora?", which was collectively liked well enough to include in the movie. The camera's anticipatory movement to catch the comment on-camera reveals that it was at least rehearsed before filming.
The remark of Thor noted that Captain America copied his beard and hairstyle is improvised by Hemsworth and was kept in the finished film.
Since the film was being shot before the release of Black Panther, the Russos didn't have the same grasp on those characters as the rest and gave the actors a lot of leeway for what they thought their characters would say. They were quite surprised by how powerful "Wakanda forever" ended up being, and according to them, the Wakandan battle cries carrying over were entirely the actors' idea.
The Russo Brothers stated in a podcast interview that the actual reason of Hulk refusing to "come out" wasn't him being scared of Thanos (while he certainly was taken back a bit by being so easily defeated), but rather Hulk simply being tired of Banner only using him when he needs him during a battle and refusing so to spite him.
Joe Russo confirmed that Valkyrie escaped with the other Asgardians in one of the ship's escape pods before Thanos attacked in the opening scene. She was also confirmed to have survived Thanos' fingersnap, as she appears in Avengers: Endgame.
The big snap left unclear the fate of the characters who did not appear in the film. Ininterviews, the Russo Brothers revealed that Betty Ross, Lady Sif, Loki's actor and the Galaga Guy died as well of Thanos's fingersnap, while Nakia, Aunt May and Howard the Duck survived. They did not want to comment on Korg, Miek, Shuri, Ned, and Jane Foster. However, Spider-Man: Far From Home director Jon Watts later confirmed after the release of Avengers: Endgame that May was indeed a casualty of the snap.
According to James Gunn, Thor is joking when he claims that the Groot-language was taught as an elective on Asgard.
The Russo Brothers confirmed the reason why Spider-Man's death dragged out longer than the other characters, was because his Spidey Senses could sense the snap starting to affect his body, triggering his healing process to slow it down temporarily. This is also why he was the last of the characters to die due to the snap.
The reason why Thanos waited years before even starting to collect the Infinity Stones was because, until he could get some kind of lead on the Soul Stone, there was no point in attracting attention to himself by going after the others.
In a Q&A for a Collider screening, the Russo Brothers confirmed that the remaining half of Asgardian refugees that were spared by Thanos were halved again by Thanos's snap.
Jeff Goldblum stated that the Grandmaster was spared, saying: "You can't kill me, I'm like a vampire."
Working Title: As mentioned above, the movies were to be called Avengers: Infinity War Part 1 and Avengers: Infinity War Part 2. Eventually only the first movie was named Avengers: Infinity War and the second went without a name until December 2018, where it was revealed to be called Avengers: Endgame.