Glengarry Glen Ross

The film depicts two days in the lives of four real estate salesmen who are supplied with names and phone numbers of leads. They use underhanded and dubious tactics to make sales. Many of the leads rationed out by the office manager lack either the money or the desire to actually invest in land.
Blake (Baldwin) is sent by Mitch and Murray, the owners of Premier Properties, to motivate the salesmen. Blake unleashes a torrent of verbal abuse on the men and announces that only the top two sellers will be allowed access to the more promising Glengarry leads and the rest of them will be fired.
Shelley "The Machine" Levene (Lemmon), a once-successful salesman now in a long-running slump and with a chronically ill daughter in the hospital with an unknown medical condition, knows that he will lose his job soon if he cannot generate sales. He tries to convince office manager John Williamson (Spacey) to give him some of the Glengarry leads, but Williamson refuses. Levene tries first to charm Williamson, then to threaten him, and finally to bribe him. Williamson is willing to sell some of the prime leads, but demands cash in advance. Levene cannot come up with the cash and leaves without any good leads.
Meanwhile, Dave Moss (Harris) and George Aaronow (Arkin) complain about Mitch and Murray, and Moss proposes that they strike back at the two by stealing all the Glengarry leads and selling them to a competing real estate agency. Moss's plan requires Aaronow to break into the office, stage a burglary and steal all of the prime leads. Aaronow wants no part of the plan, but Moss tries to coerce him, saying that Aaronow is already an accessory before the fact simply because he knows about the proposed burglary.
At a nearby bar, Ricky Roma (Pacino), the office's top "closer," delivers a long, disjointed but compelling monologue to a meek, middle-aged man named James Lingk (Pryce). Roma does not broach the subject of a Glengarry Farms real estate deal until he has completely won Lingk over with his speech. Framing it as an opportunity rather than a purchase, Roma plays upon Lingk's feelings of insecurity.
The film then skips to the next day when the salesmen come into the office to find that there has been a burglary and the Glengarry leads have been stolen. Williamson and the police question each of the salesmen in private. After his interrogation, Moss leaves in disgust, only after having one last shouting match with Roma. During the cycle of interrogations, Lingk arrives to tell Roma that his wife has told him to cancel the deal. Scrambling to salvage the deal, Roma tries to deceive Lingk by telling him that the check he wrote the night before has yet to be cashed, and that accordingly he has time to reason with his wife and reconsider.
Levene abets Roma by pretending to be a wealthy investor who just happens to be on his way to the airport. Williamson, unaware of Roma and Levene's stalling tactic, lies to Lingk, claiming that he already deposited his check in the bank. Upset, Lingk rushes out of the office, and Roma berates Williamson for what he has done. Roma then enters Williamson's office to take his turn being interrogated by the police.
Levene, proud of a massive sale he made that morning, takes the opportunity to mock Williamson in private. In his zeal to get back at Williamson, Levene accidentally reveals that he knows Williamson lied to Roma minutes earlier about depositing Lingk's check and had left the check on his desk and had not made the bank run the previous night â something only a man who broke into the office would know. Williamson catches Levene's slip of the tongue and compels Levene to admit that he broke into the office. Levene finally caves in and admits that he and Moss conspired to steal the leads. Levene attempts to bribe Williamson to keep quiet about the burglary. Williamson scoffs at the suggestion and tells Levene that the buyers to whom he had made his sale earlier that day are in fact bankrupt and delusional and just enjoy talking to salesmen. Levene, crushed by this revelation, asks Williamson why he seeks to ruin him. Williamson coldly responds, "Because I don't like you."
Levene makes a last-ditch attempt at gaining sympathy from Williamson by mentioning his daughter's health, but Williamson cruelly rebuffs him and leaves to inform the detective about Levene's part in the burglary. Roma walks out of the room as Williamson enters. Unaware of Levene's guilt, Roma talks to Levene about forming a business partnership before the detective starts calling for Levene. Levene walks, defeated, into Williamson's office. Roma then leaves the office to go out for lunch, while Aaronow returns back to his desk to make his sales calls as usual.

Question: Who  helps the police question each salesman in private?
Answer: Williamson