Hello Saurabh, 

Not all IITs conduct interviews anymore, it varies from institution to institution and depends on their choice. COAP is a relatively new attempt by the IITs to helps students ( mainly M. Tech)  retain seats in the IITS. COAP generally has a total of 5 rounds and all deserving students are allowed seats by the end. Specific IITs that you may attempt to get admitted into through COAP may conduct counselling or interview based on their choice. 

Hope this helps :)