Use 'undercurrent' to describe feelings not visible on the surface, but influencing behavior subtly. Amidst the laughter, there was an undercurrent of tension among them.
Pro Tip 2/3
Contrasting Appearance
'Undercurrent' implies a difference between what is shown and what is felt or intended. His smile didn't reach his eyes, revealing an undercurrent of sorrow.
Pro Tip 3/3
Nuanced Context
Look for subtle narrative cues to detect an 'undercurrent' in literature and speech. The dialogue was casual, but an undercurrent of urgency ran through it.