We only mention Henry Costin because Robert Pattinson plays him in the movie. Grann writes little about Costin, a British corporal who accompanies Fawcett on the expedition with James Murray. Murray is such a loud and colorful character that Costin fades into the background. Partly for that reason, though, his levelheaded letters and journals turn out to be an invaluable resource for Grann.
Although Costin gets ill, he lives. He's one of Fawcett's most trusted companions; in fact, Fawcett describes him as one of "the only assistants I could ever call completely reliable and fully adaptable, and never have I wished for better company" (12.67).
One moment you should note: during this expedition, Costin is "bitten by vampire bats" (12.30). That alone makes Pattinson the perfect casting choice.