They'll eat donuts, lounge around in the break room, and once in a while, they'll write a report. If someone comes in with a tip, unless it's extremely detailed and thorough, chances are they'll read it twice and then toss it, or they'll pass it on to a field office, which will pass it on to a resident agency, and then after six months they'll start an investigation and write a report. Nobody will read it. When something does happen, someone will write a report on why they forgot the consider the findings of the earlier report. Nobody will read this report either.
Like any other bureaucracy, at least three-quarters of what the FBI does is paperwork. Thanks to TV, most normalfags have the idea that every FBI agent is trained in long-range sniping and close-quarters fighting and knows 2 computer languages and 3 martial arts and can speak 4 languages. Also in Hollywood, concepts like "federal jurisdiction" or "probable cause" never seem to exist.