Short answers, pulled from the story.
Tom Hiddleston learned that Loki would die in May 2016 when he met with directors Anthony and Joe Russo. The directors informed him of this decision before production began, which fundamentally altered the tone of the entire franchise.
Avengers: Infinity War was shot entirely with ARRI Alexa IMAX 2D cameras. This marked the first time a Hollywood feature film was shot entirely with IMAX digital cameras to match the visual scale of the narrative.
Thanos sacrificed his adopted daughter Gamora to acquire the Soul Stone on the planet Vormir. The Stone's keeper Red Skull told Thanos he could only obtain the stone by sacrificing someone he loved, and Thanos chose to save Nebula from torture.
Mark Ruffalo described the Hulk in the film as having the mental capacity of a five-year-old. This was a departure from his usual rage-fueled persona, and the Hulk refused to appear for much of the film.
The final snap disintegrated half of all life, including Barnes, T'Challa, Groot, Wanda, Wilson, Mantis, Drax, Quill, Strange, Parker, Maria Hill, and Nick Fury. Nick Fury was the last to disintegrate after sending an emergency signal on a modified pager.