Like many a thriller WSJ Crossword Clue

Think of a tightly wound string that keeps you on edge.

???? means stretched or pulled tight, not slack, and can also refer to something that is tense or emotionally strained.

The word '????' is often used to describe something that is tightly drawn or tense, which is a common characteristic of thriller movies or books. Thrillers are designed to keep the audience on the edge of their seats, creating a sense of tension and suspense, hence they are '????.'

Like a tug-of-war rope, Like an archer's bowstring before a shot, Stretched with no slack, Like a well-crafted thriller, Full of suspense