Why does Nag keep Rikki engaged in conversation when they first meet?

When Rikki and Nag first meet near the beginning of "Rikki-Tikki Tavi," the tailorbird named Darzee has just told Rikki about how Nag ate one of his babies the day before. Rikki has just recently been rescued and does not know who Nag is, which prompts Nag to appear. Nag, a cobra, fears mongooses like Rikki, as mongooses are capable of killing cobras. As a result, Nag hopes to keep Rikki engaged in conversation while his wife Nagaina approaches in the bushes. Their goal is for Nagaina to attack Rikki when he is not expecting it. Attacking Rikki by surprise is the cobras' best chance of killing him. Darzee notices Nagaina in the grass, though, and is able to warn Rikki in time for him to take action.

He jumped up in the air as high as he could go, and just under him whizzed by the head of Nagaina, Nag's wicked wife. She had crept up behind him as he was talking, to make an end of him. He heard her savage hiss as the stroke missed.

Darzee's warning allows Rikki to get out of the way of Nagaina's strike in time.