Standing before an absolute ruler like the King of Persia carries a constant, underlying threat of death. In such a royal court, even the slightest change in a servant's facial expression can lead to disastrous consequences.
The king notices a distinct shift in Nehemiah's appearance, pointing out that he looks unwell despite not suffering from any physical illness [רש״י, ביאור שטיינזלץ]. His face projects a deep sense of brokenness, resembling the appearance of a sick man [אבן עזרא]. This visible change was the natural result of the fasting and self-affliction Nehemiah had recently taken upon himself. Alternatively, this shift in his appearance was an act of divine providence, deliberately designed to catch the king's attention and create an opportunity for a crucial conversation [מלבי״ם].
Observing this, the king concludes that Nehemiah is harboring something dark in his heart. The primary approach among commentators is that the king immediately suspected an assassination attempt. Because Nehemiah was the one serving his wine, the king assumed he intended to poison the cup, interpreting his tense expression as the typical nervousness of a murderer [רש״י, מצודת דוד, חומת אנך]. Conversely, other commentators suggest the king spoke out of genuine care and affection. In this view, the king simply recognized that Nehemiah was burdened by heavy thoughts and deep worries [רלב״ג, ביאור שטיינזלץ, חומת אנך בשם אבן יחיא].
Hearing the king's remark, Nehemiah is struck with immense fear, driven by the immediate danger to his life. According to the view that the king suspected treason, Nehemiah was terrified of being framed for betrayal, knowing that a ruler's mere suspicion was enough to warrant an immediate execution [רש״י, מצודת דוד, ביאור שטיינזלץ]. Even if the king was merely expressing concern, Nehemiah knew that displaying sadness in the royal presence was itself considered an offense. He worried that the king's initial inquiry could swiftly turn into deadly anger [רלב״ג, מלבי״ם]. Another perspective suggests his fear was more personal. He worried that if he exposed his true pain and presented a massive request, such as rebuilding the walls of Jerusalem, the king would simply mock him [חומת אנך בשם אבן יחיא].