Henry Crumpton’s fascinating book about his time working for CIA includes an account of his time in 1998 and 1999 seconded, as a CIA liaison, to a senior post in the FBI.   This account has a profound reflection of the cultural differences between the two organizations, differences which arise from their different primary missions: for FBI, the mission is to solve criminal cases (which is essentially a backwards-looking activity played against a suspect’s lawyers) versus, for CIA,  the identification and avoidance of potential threats (which is inherently forwards-looking and played for an intelligence client).  Crumpton summarized his reflections (contained on pages 112-115 of his book) in a Politico op-ed column here:

First, the FBI valued oral communications as much as or more than written. The FBI’s special-agent culture emphasized investigations and arrests over writing and analysis. It harbored a reluctance to write anything that could be deemed discoverable by any future defense counsel. It maintained investigative flexibility and less risk if its findings were not written — or at least not formally drafted into a data system. Its agents were not selected or trained to write.
This is also tied to rank and status: Clerks and analysts write, not agents. Agents saw writing as a petty chore, best left to others.
In contrast, most CIA operations officers had to write copiously and quickly. To have the president or other senior policymakers benefit from clandestine written reports — that was the holy grail. CIA officers prized clear, high-impact written content.
The second major difference between the FBI and CIA was their information systems. The FBI did not have one — at least one that functioned. An FBI analyst could not understand a field office’s investigation without going to that office and working with its agents for days or even weeks. With minimal reporting, there was no other choice.
CIA stations, in contrast, write reports on just about everything — because without written reports, there was no intelligence for analysts and other customers to assess. The CIA required high-speed information systems with massive data management, and upgraded systems constantly.

The third difference was size. The FBI was enormous compared with the CIA. The FBI personnel deployed to investigate the East Africa bombings, for example, outnumbered all CIA operations officers on the entire African continent. The FBI’s New York field office had more agents than the CIA had operations officers around the world. The FBI routinely dispatched at least two agents for almost any task. CIA officers usually operated alone — certainly in the development, recruitment and handling of sources.

A fourth difference was the importance of sources. While both the FBI and the CIA placed a premium on a good source, the FBI did not actively pursue them beyond the context of an investigation. The agents would follow leads and seek a cooperative witness or a snitch, often compelled to cooperate or face legal consequences.
FBI agents seldom discussed sources. When they did, it was often in derogatory fashion. But they discussed suspects endlessly. That was their pursuit. And for the FBI, sometimes sources and suspects were one and the same.
CIA officers, on the other hand, routinely compared notes and lessons learned about developing, recruiting and handling sources — though couched so that specifics were not revealed. Ops officers’ missions and their sense of accomplishment, even their professional identity, depended on the success of sources and the intelligence they produced. FBI agents wanted evidence and testimony from witnesses that led to convictions and press conferences.
A fifth difference was money. The FBI had severe limitations on how much agents could spend and how they could spend it. The process to authorize the payment of an informant or just to travel was laborious.
As a CIA officer, however, I routinely carried several thousand dollars in cash — to entertain prospective recruitment targets, compensate sources, buy equipment or bribe foreign officials to get things done. I usually had to replenish my well-used revolving fund every month.
When I told FBI agents this, they seemed doubtful that such behavior was even legal. I often had to explain that the CIA did not break U.S. laws — just foreign laws.
Sixth, the FBI harbored a sense that because it worked under the Justice Department, it had more legal authority than the CIA. Some, after a few drinks, expressed moral objections to the CIA’s covert actions. I would argue that covert action, directed by the president and approved by congressional oversight committees, is legal. But somehow the notion of breaking foreign laws seemed less than ideal to some of my FBI partners.
Seventh, the FBI loved the press and worked hard to curry favor with it. For the CIA’s Clandestine Service, the media was taboo. Most of us had experienced occasions when media leaks undermined operations. Sometimes, our sources died because of this coverage. On top of that, we felt that the media seemed intent on portraying the CIA in a negative light. A CIA operations officer avoided the press like the plague.

For the FBI, it was the opposite. Positive press could help fight crime and boost prestige and resources. Every FBI field office worked the media.

Eighth, the FBI collected evidence for its own use, to prosecute a criminal. The CIA primarily collected intelligence for others, whether a policymaker, war fighter or diplomat. The FBI, therefore, lacked a culture of customer service beyond the Justice Department. Without a customer for intelligence, the CIA had no mission.
Ninth, the FBI’s field offices, especially New York, acted as their own centers of authority, even holding evidence, because of their link to the local prosecutor. A city district attorney and civic political actors had great influence over an investigation.
The CIA station instead had to report intelligence to Langley, because the incentive came from there and beyond — particularly the White House.
Tenth, the FBI worked Congress. Every FBI field office had representatives dedicated to supporting congressional delegates. The FBI also had the authority to investigate members of Congress for illegal activity. So the bureau had both carrots and sticks.
But the CIA, particularly the Clandestine Service, had minimal leverage with Congress. Most CIA officers engaged Congress only when required to testify.”

The consequences of these differences for us all are immense.  Crumpton again:

Reference:
Henry A Crumpton [2012]:  The Art of Intelligence:  Lessons from a Life in the CIA’s Clandestine Service. (New York, NY, USA:  Penguin Books).