Final answer:
The interview skill in use when noticing non-verbal cues indicating pain that Mr. Henderson denies verbally is observing. This skill is important for healthcare professionals to identify true patient conditions.
Step-by-step explanation:
The interview skill that is being used when noticing Mr. Henderson's grimace upon standing and his slow, careful movements, despite his denial of pain, is observing. Observational skills are crucial in healthcare settings, allowing healthcare professionals to detect signs of discomfort, stress, or illness that patients may not verbally communicate. By observing non-verbal cues such as facial expressions and body movements, healthcare providers can gather valuable information about a patient's condition.