Abbas Milani highlights the fascinating life of Ayatollah Montazeri, the once-close pupil and presumed successor to the revolution's first Supreme Leader, Khomeini:
Tensions between Khomeini and Montazeri began when someone on Montazeri's staff leaked the story of secret deals between Iran and the United States--what turned out to be the Iran-Contra Affair. Khomeini executed the staffer, despite protestations from Montazeri. A few months later, as the nation learned of Khomeini’s ill health, Montazeri learned of mass executions in prisons on the order of Khomeini. Prisoners serving time on earlier charges were to be retried--in procedures often lasting no more than a few minutes--and executed if found to be still opposed to the regime. Instead of keeping a pragmatic silence and awaiting Khomeini’s death, as many of his advisors recommended, Montazeri wrote a harshly worded letter to Khomeini condemning the orders, saying that this is not the kind of revolution they had fought for together. This time, the price for protesting murder and moral perfidy was the direct wrath of Khomeini.
More here. The photo above features (from left to right) Rafsanjani, Khomeini, and Montazeri.