Both are the extreme cases. One in which the Prime Minister can do whatever he wishes and the second case in which the Prime Minister has to consult other leaders and parties are not totally correct.
If a Prime Minister is free to do as he wishes, there is a possibility that he might develop authoritarian or dictatorial tendencies.
On the other hand, if all the time he has to consult other leaders and parties, it would hamper his working. Therefore, he should have freedom of taking decisions, but he should also consult with other leaders of parties on important issues.