For more than a quarter of a century, he has worked behind the scenes to help one of the country’s biggest universities tackle its most tangled legal problems.

Now his place of employment is changing. On February 1, the highly regarded general counsel and professor of criminal procedure at New York University, S. Andrew Schaffer, trades his homey corner office at the Elmer Holmes Bobst Library off Washington Square for the austere concrete of 1 Police Plaza.

As the New York Police Department’s newly appointed deputy commissioner of legal matters, Mr. Schaffer will be the department’s top lawyer, a relatively unseen post but a critical one, for it sets the tone and policy for the rank and file of the city’s gargantuan police force.

In an interview this week with The New York Sun, Mr. Schaffer declined to talk about the upcoming challenges, his professional and moral attitudes toward the police, or anything regarding the police in general. His goal, he said, is simple: “to be the very best lawyer I can be for my client.”

His responsibilities will increase exponentially. Instead of supervising a team of nine attorneys at NYU on matters such as real-estate contracts and bonding issues, Mr. Schaffer, an active hiker and Sunday morning basketball player at age 63, will now be responsible for commandeering a battalion of 65 attorneys and a staff of 130. He will coordinate regularly with the city’s primary attorney, Michael Cardozo, the corporation counsel, and act as legal consigliere to the police commissioner, Raymond Kelly.

“I wanted a heavyweight, and that’s what I got,” Mr. Kelly said of the appointment. “When you have guy like Andy Schaffer on board, that brings instant credibility, stature, and a certain gravitas.”

Mr. Schaffer’s appointment comes at a critical time for the Police Department, which under Mr. Kelly has attempted to streamline itself into a more effective and flexible agency despite budget constraints, a dwindling police force, and heightened terror alerts since September 11. While Mr. Kelly has earned high marks for his hands-on approach to crime-fighting, anti-terrorism tactics, and his ability to keep the total reported major crimes headed down year after year, the department has come under criticism by civil-liberties groups.

The issues are myriad.

One continuing legal battle has been over the department’s videotaping of political demonstrations. Attorneys for the police have argued that the videotapes assist in training for crowd-control scenarios, while attorneys for civil rights groups say the videotapes could be used to create a database in which the faces and images of protesters would be stored and catalogued.

Another legal fight still rages over the historic number of arrests made last summer during the Republican National Convention, and the detention of protesters at a bus depot.

There are also internal issues, such as proper protection for whistleblowers, the treatment of members of minority groups, and the increasing cost of lawsuits filed against the department.

According to the most recent annual audit of the department conducted by the city comptroller’s office, lawsuits stemming from police misconduct, false arrest, excessive force, and other factors such as motor accidents, police injuries, and civil-rights claims cost the city a total of $78.7 million in fiscal year 2002, a figure that averages out to more than $6.5 million a month.

The annual cost, which often includes complaints and lawsuits filed in previous years, has been steadily rising for the past decade and has nearly doubled since 1996.

A spokeswoman for the comptroller, Yvette Jackson, said the most expensive and common type of legal action against the police department was the civil-rights discrimination case, which includes claims filed by external groups, such as political protesters, and claims filed by internal groups, such as minority officers with equal-opportunity issues. The exact figures for civil-rights payouts were not immediately available, Ms. Jackson said. Speaking of Mr. Schaffer, the executive director of the New York Civil Liberties Union, Donna Lieberman, said: “He certainly has his work cut out for him.”

Mr. Schaffer said he never saw the job coming. Indeed, he said that in his years acting in a variety of roles, such as defending indigents as a student at Harvard Law School, looking to reform criminal procedures at the nonprofit Vera Institute for Justice, and even working as a prosecutor in the U.S. attorney’s office, Mr. Schaffer said in an interview with the Sun that he never could have imagined working for a client like the Police Department.

When Mr. Kelly came to see Mr. Schaffer at his NYU office the day before Thanksgiving, Mr. Schaffer said he thought “they were going to send a SWAT team in to make arrests and he was coming to give me a personal courtesy.”

Instead, the commissioner made a job offer. Mr. Schaffer answered in a characteristically cautious, reflective way.

“My answer is not no,” he told Mr. Kelly.

Research was then conducted. Over the next few days, Mr. Schaffer was on a personal fact-finding mission and traveled to meet with the department’s recently retired top lawyer, Stephen Hammerman, 67, a former vice-chairman of the board at Merrill Lynch, who now lives in Arizona. After pondering the duties, nuances, and politics of the job, Mr. Schaffer said, he decided to take it because it was nothing he had ever done before. Besides, he said, “The most important service in life is public service.”

His appointment, according to former deputy commissioners and other notable attorneys around the city, is a promising and noteworthy one.

“With Andy, you get the best of all worlds,” one former deputy commissioner, the president of the John Jay College of Criminal Justice, Jeremy Travis, said.