How did Khomeini rise to power in Iran?

Answer 1
This is a controversial issue in Iran. The regime would like the people to think that it was in fact the people and their support that brought Khomeini back and ended the tyranny of the Shah. That sounds very vain though as at the time of the revolution Khomeini was living in exile in France. The controversial issue for many Iranians is how was it that at the age of no internet, no satellite and no mobile phones, how was it that Khomeini's speeches and messages were delivered in the form of 'shabnameh' (night letters) to almost everyone's house every night?

The strongest debate going on in Iran now is that Khomeini and the Islamic revolution was aided by the west in an attempt to over turn the Shah. The Shah had apparently stopped taking orders from his patrons, in his last speech he held up a bottle of wine promising his people that in the near future he would be selling their oil in bottles instead of barrels. Some believe that was when the west decided they had to over turn him.

BBC was the surrogate mother of the Iranian Islamic Revolution by broadcasting every message, letter and speech of Khomeini from France. US told the Shah to leave the country when the protests were getting stronger and then Khomeini was brought back to Iran.

It seems Khomeini and his party outsmarted the countries that helped them to power. What they said before the revolution was completely different from the actions they took after it. Even the people who had fought and voted for the revolution were now unsatisfied. But then a big mistake triggered by the west united the whole country: Iraq attacked Iran one year after the revolution backed by western powers who provided the Arab country with arms and weapons.

Iranians, being very patriotic, forgot/ignored everything else to protect their country. The result was that after the 8 year war had ended, the Islamic regime was too well established to be changed and reformed. The people had been tricked into paying a very expensive price to be under another tyrannical regime. This one worse than the previous one as this one was using religion to justify every brutal action.

Answer 2
Shah had violated the constitution, condemned the spread of moral corruption in the country and submitted himself to the will of America and Israel.
On the afternoon of 'Ashoura (June 3, 1963), Ayatollah Khomeini delivered a speech at the Feyziyeh Madrasah denouncing the Shah as a "wretched miserable man", and warning him that if he did not change his ways, the day would come when the people would offer up thanks for his departure from the country.

On June 5, 1963, Imam Khomeini was arrested. This sparked three days of major protests throughout Iran and led to the killing of some 400 pro-Khomeini protesters. The Imam was kept under house arrest for 8 months and he was released in 1964.

In November 1964, Ayatollah Khomeini denounced both the Shah and the United States, this time in response to the "capitulations" or diplomatic immunity granted by the Shah to American military personnel in Iran. In November 1964, Imam Khomeini was re-arrested and sent into exile.
In early 1970 Khomeini gave a series of lectures in Najaf on Islamic Government, later published as a book titled variously Islamic Government or Authority of the Jurist (Welayat al-Faqih).

Imam Khomeini spent over 14 years in exile, mostly in the holy city of Najaf, Iraq. Initially he was sent to Turkey on 4 November 1964 where he stayed in the city of Bursa for less than a year. Later in October 1965 he was allowed to move to Najaf, Iraq, where he stayed until being forced to leave in 1978, after then-Vice President Saddam Hussein killed his son Mustapha and forced the Imam out after which he went to Neauphle-le-Ch√Ęteau in France where he stayed for four months.

Imam Khomeini became the most influential leader of the opposition to the Shah perceived by many Iranians as the spiritual, if not political, leader of revolution. As protest grew so did his profile and importance. Although thousands of kilometers away from Iran in Paris, the Imam set the course of the revolution, urging Iranians not to compromise and ordering work stoppages against the regime.

He sent letters to Iran which were copied and distributed among people.
Imam Khomeini is known to be a revolutionist to overthrew the Shah's monarchy by "pen and paper"