Interface text is usually written last, by whoever is building the screen, and it does more work than any other element.

Rules that hold up

• Label buttons with the outcome, not the mechanism. "Save draft" and "Publish" tell you what you get; "OK" tells you nothing.
• Say what will happen next, especially before something irreversible. "This cannot be undone" belongs next to the button, not in a paragraph above it.
• Write errors as instructions. "Password must be at least 12 characters" is useful; "Invalid password" is a locked door.
• Never blame the user. "We could not find that account" is the same information as "you entered the wrong email" without the accusation.

Where it matters most

Anywhere money, deletion, or permanence is involved. Users read carefully in exactly those moments, and vague language there reads as evasion — which costs trust that the rest of the interface has to work to earn back.