The doctrine of Separation of Powers was a theory propounded by Montesquieu, a political philosopher and an activist in French Revolution.
According to this principle, the three powers of a state – Legislature, Executive and Judiciary should remain three independent bodies and act separately. It is a very good instrument to avoid tyranny as the vesting of these three powers in one organ, will lead to abuse of law and human rights violation.
Later on James Madison, Father of the American Constitution, imbibed the doctrine of Separation of Powers in their Constitution and they strictly adhere to this principle. In India we adopted this doctrine from the American Constitution, but it is not strictly enforced as in U.S.A.
The legislature, executive and judiciary are three independent bodies but there are many functions which make them intermingle with each other. The doctrine of separation of powers remains controlled by checks and balances in India.
For e.g.,- The executive which consists of President, Vice-President, Prime Minister and the Council of Ministers remains responsible for every action they take to the legislature which remains the law making the body of India. Legislature consists of Lok Sabha and Rajya Sabha. Now, whatever bills passed by the legislature, the executive must give assent for the bill be a law.
Whatever laws executed by the Executive must pass through the scrutiny of Judiciary to ensure that the laws remain within the parameters of constitutional principles of India. If the law violates constitutional principles, then the judiciary has the power to make that law void ab initio or void from the outset.
So you could see that even though, they work independently, there are many checks and balances implemented by each body of the government in India. India being a huge and diverse country, rigid principles cannot be enacted over here. In fact Indian Constitution is also known as flexible constitution as it got amended more than 100 times.