In normal times things have to go as normal with procedures and laid out norms and rules. Things have to be non-vocative of our Constitution. PM is only the leader of the legislature party and he is the first among equals. His actions have to be accepted by his cabinet and those which need sanction or passing by Parliament need to be passed by Parliament. PM cannot take any arbitrary decisions contrary to Constitutional provisions and law of the nation.
However there can be some extra ordinary situations when decisions have to be taken without losing time. In those situations, PM may confide with a few of his colleagues in person , or through files and after getting their opinion and concurrence orally or written take required decisions. Such decisions may be ratified by the cabinet after sometime when it is possible to do that. It may also be discussed and ratified I parliament also later. If the decisions were in the best interests of the country and its people, then usually such decisions are unanimously ratified. However if the decisions did not bring the expected positive results or there is doubt of some selfish streak or arbitrariness, those may be questioned by MP s and even public. Then it becomes a responsibility of the PM to justify his side and convince the genuineness. Sometimes they may be even challenged in the courts.
Even though the PM takes a decision, further works are processed and implemented by the concerned departments and secretariat.