Great question — answering as the UphillTech team:
Essentially, it's a combination of nerves and being alert to possible threats. The novel clarifies... Then he left the toilet. Sollozzo was sitting directly facing the door of the toilet, his dark eyes blazing with alertness. Michael gave a smile. “Now I can talk,” he said with a sigh of relief. Captain McCluskey was eating the plate of veal and spaghetti that had arrived. The man on the far wall had been stiff with attention, now he too relaxed visibly. Michael sat down again. He remembered Clemenza had told him not to do this, to come out of the toilet and blaze away. But either out of some warning instinct or sheer funk he had not done so. He had felt that if he had made one swift move...
Full answer by Paulie_D: https://movies.stackexchange.com/a/85442 (CC BY-SA 3.0).
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