“suddenly”

In a sharp, unexpected manner with no warning or transition.

“The music stopped abruptly, leaving the dancers frozen mid-step.”

Best for: A clean upgrade from 'suddenly' in most prose or journalistic writing.

Occurring at once, without any perceptible lapse of time.

“The bridge collapsed instantaneously, swallowed by the fog below.”

Best for: Works well in technical, dramatic, or scientific contexts for emphasis.

With alarming, headlong suddenness, as if falling off a cliff.

“Her confidence dropped precipitously the moment he entered the room.”

Best for: Best used when the sudden change carries a sense of danger or steep decline.

Used in the phrase 'bolt upright' or alone to convey electric, immediate action.

“She sat bolt upright at 3 a.m., certain she had heard her name.”

Best for: Punchy and visceral — ideal for thriller, horror, or action-driven fiction.

In an archaic sense, immediately and without restraint or delay.

“He incontinent quit his post and vanished into the countryside.”

Best for: A rare archaic usage that startles readers — reserve for historical fiction or irony.

At once; immediately; without the slightest delay.

“The judge demanded the documents be produced instanter.”

Best for: Rooted in legal Latin — striking in formal, historical, or deliberately archaic prose.