The opening moments of 1917 are very clear in laying out the mission that our two leading characters must embark on.
Young soldiers Schofield and Blake are tasked with delivering a message to a nearby battalion, which, if they are successful, will save countless British lives. Blake also has personal stakes in this quest, as his older brother is among the men who will be saved if they reach their destination on time and everything goes to plan.
While Blake and Schofield are on even footing in terms of screen time and dialogue, the familial tie Blake has to the mission makes you believe that he's the main protagonist, that he'll see the movie through to the end and reunite with his brother. Predictable as that would be, that's exactly what would happen in most movies.
And so, it was a bit of a shock when Blake was stabbed by a German soldier quite early in the film, causing him to bleed out in Schofield's arms.
Not only did this twist kill off the guy who we thought was the hero, but it also completely destroyed the happy "family reunion" ending we all had in our heads. It also sent a clear message that nobody was safe in this movie - not even the main characters - a clever way to keep viewers invested in Schofield's journey, making audiences worried for his life whenever he encountered any enemy forces.