Finger on lip is giveaway for formulating a lie. That it is one finger, the pointing finger, hints that the lie he’s thinking of responding with is an accusatory one. Just my opinion.
If his hand had been over his chin it would indicate thoughtful consideration of what he’s hearing, but covering his mouth with a finger implies tension, likely a subconscious hesitation brought on by fear the next words will go over as false to discerning listeners. He’s thinking more about his response than about listening to the question. But what do I know...