Possible Clues:

• Stare at with open mouth
• Look stupid, in spending a penny
• Show fixation, maybe
• Exhibit astonishment
• React with disbelief
• Stare in wonder, say, welcoming old man in retirement
• 'Um, what?' expression
• Show great surprise
• 'I'll speak to it though hell itself should ____' (Hamlet, on his father's ghost)
• Drop jaw