In early versions of Word, the Spell Check feature was something that you explicitly invoked, and then you had to sit and wait while the program looked for all your potentially-misspelled words, and then showed them to you one at a time for a decision on what to do for each one. Word did introduce an Auto Spell Check feature to run spell check when the user was idle, so that when you hit the Spell Check button, the results were ready to go. However, the Auto Spell Check was still a blocking operation. As a result, a lot of users turned it off because it always seemed to decide “Now would be a good time to spell-check the document” just as you wanted to do something, forcing you to wait for the spell check pass to complete before you could, say, save and exit.

Tony [Krueger] made the spell checker much more unobtrusive so that it didn’t interfere with your foreground work. And when it found a problem, instead of waiting for you to trigger a spell check, it immediately drew red squiggles under potentially-misspelled words (and later green squiggles under potential grammatical errors).