Unbidden

Then an unbidden guest stepped to the microphone.

The thoughts came back, unbidden, to torture Tom's guilty conscience.

However, it’s difficult to control our reactions to stressful situations, such as when your supervisor at work tells you to produce something on a deadline you feel unable to meet. The shortness of breath, the sweaty palms, the adrenaline surge, the sense of wariness and unease come unbidden.