Why can't I change a film block's words on the page?

Because they come from the film, not from the page. The same film feeds several blocks across several pages, so a change typed on one page would have to quietly change all the others. You change it once on the film, under Pages & Content, Films, and every page follows.

Open a film block in the website builder and its settings say which part of the film fills it, with a button straight to that tab of the film the block is showing: Details for Film Hero and Film Details, People & Credits for Film Credits, Festivals & Awards, Screenings, Critic Quotes, Press Articles, Where to Watch, or Press Kit. The block's own settings, which film it shows, its heading and how it is displayed, stay on the page.

The Film box at the top of a film block is normally left alone. It reads The main film, with that film's name, which means the block shows whichever film you have set as the main film, so your website follows if you ever change which that is. Choose a film in the box to pin the block to that one film instead. On a block every kind of website shares (Events, Reviews and Press Quotes, People, Downloads) the box reads No film, and the block then uses the items you type into the block itself.