Bill Arnold, who is apparently a significant figure in Sacramento (thus, presumably, a politician or reporter), is a slightly drunk new acquaintance of Stephanie Dobbs, who effectively accosts her on the California Bullet Train. He's already on Chance's radar, having been fiddling constantly with his jacket pocket while watching Stephanie from afar, and indeed from a bar, and he plonks himself down next to her to apologize from his past, negative comments about her work just as her drink is delivered. Chance notices, amid Bill's self-aggrandizing comments, that, uniquely among those present, Stephanie's drink contains ice cubes, everyone else having been served crushed ice, and steers her away before she can drink it.
Apparently an opportunist in all aspects of life, Bill decides not to pass up free alcohol, and swigs her untouched beverage - his last mistake in this life, as it was poisoned by the assassin masquerading as a barman.