Avengers: Endgame - What You Definitely Need to Remember (Spoiler Free)

Oh, how I cried. Never have I cried over a movie as much as I have here. Definitely an experience not just MCU fans, but people in general should have.
I will not be reviewing Avengers: Endgame yet, as I will be seeing it again soon, and am still processing. Needless to say, my emotions are all in a jumble, I feel as though my heart had been pulled out of my chest and crushed before me, and my hands still hurt from all the clapping I did opening night.
It goes without question that Infinity War is critical for this movie, but will repeat it just in case anyone is thinking of watching Endgame without having ever watched Infinity War. Trust me, you'll be thankful.
Captain America: Civil War, because that rift between Iron Man and Cap is critical
Next, rewatch, or at least keep in mind the first Avengers film, Thor: The Dark World, and Guardians of the Galaxy. They play a critical role. If you can't do all three, watch refresh yourself on the attack of New York in Avengers, the attack of the Dark Elves on Asgard in The Dark World, and Peter Quill's dance as he heads towards the power stone at the beginning of GotG.
You may also want to rewatch Doctor Strange, Captain Marvel, Ant-Man and the Wasp for characters and the quantum realm. Watch Avengers: Age of Ultron for that Mjolnir scene.
Iron Man 3 if you want to recognize a certain someone in the film, Captain America: The First Avenger if you need a refresher on Peggy.
Now, if you don't have any association with any Marvel characters outside of the big join-ups, you may want to look up some info. At the showing I went to, someone was laughing at a very serious, sad moment, the rest of the audience crying. I don't know if this person didn't know the backstories of certain characters, but please, if you don't understand something during a serious, sad moment where everyone is crying, don't laugh. It hurts us MCU fans, and most likely casual fans as well - to have such a moment spoiled by a disrespectful viewer laughing during such a moment. I don't want to spoil the movie, but it is near the end, over something that happens to one of the trinity.
If you haven't watched Agent Carter, you might miss out on a reference. While I personally have not watched all of Agent Carter, I did watch enough to fully appreciate this key moment. In summary, Agent Carter follows Peggy Carter, who we all know knew Howard Stark. Howard Stark had his own Happy-like guy (Tony's Happy I mean), Jarvis. Keep a look out, basically, or at least pay attention to minor dialogue.
I have not read any comics but one comic revelation was so massive that even non-comic book fans heard of this. In Captain America's story line, in a surprising turn of events, Captain America says, "Hail, Hydra," establishing Cap as a hydra agent all along. We all know that the MCU does not follow the comic books to the letter, or even to the page, but you should keep this piece of info in your mind for a special treat.
You may or may not want to keep track of the actors plans in terms of the MCU. We all know that Endgame's purpose is to reverse The Snap, but there are serious consequences. Whether you track MCU plans or not, it is nice to try to guess after the fact how certain upcoming projects will play out.
And forget all of what you know about time travel. I am a big Doctor Who plan, and was worried this might interfere with my enjoyment of the movie, as we all know there will be some time travel, but that is completely taken off the table.
I'll do more on Endgame in a bit, but for now, prep yourself if you haven't already gone to see Endgame. Believe me, it is such a unique experience, you won't forget it.