I guess John’s not the only one who clenches his hands when he gets nervous…
I’m not an expert, so don’t take my word as gospel; this is just what a few hours of research on body language turned up.
Interwoven, clenched fingers: Frustration, restraint, disagreement, hiding a negative opinion, anxiousness.
Clasping/squeezing hands: Self-pacifying gesture as a sign of discomfort, nervousness, impatience, or even fearfulness. Increases in frequency and force corresponding stress.
Fidgeting: Tension, nervousness.
Hands over groin: Insecurity, submission, nervousness.
Holding head high: Hiding insecurity/hurt feelings.
Limited eye contact: Insecurity, defensiveness.
Obviously John must have picked up on these things, because he felt the need to try to make Sherlock feel better about the situation. Unfortunately, it was with LIES! LIES!