Some drug traders first saw him as a heroin smuggler in Thailand. Others met him as a cocaine buyer in Buenos Aires. And others as a gunrunner in Buffalo. He can recall how they all looked into his eyes, trusted him and, as a result, went to jail. Michael Levine, a special agent of the Federal Drug Enforcement Administration ... is described by his agency as an expert in deep cover" - an agent who assumes invented characters to penetrate underworld organizations. Deep cover means living among criminals for weeks or months at a time, unable to return home or admit to anyone one's real name. After playing the part of underworld figures for the Federal Government for 21 years, Mr. Levine now works as a supervisor in the D.E.A.'s New York office. Deep cover specialists play a crucial role in the D.E.A.'s long-term narcotics investigations, as well as in major investigations of the Federal Bureau of Investigation, the Bureau of Alcohol, Tobacco and Firearms ... and some local police departments. "You have to divide your brain in half," Mr. Levine said. "This half is the character you are playing. This half is always a Federal agent recording the details. If the half that is a Government official becomes inefficient, it can cost you your life." Over the years, [Mr. Levine] has been stationed in New York, Washington, Miami, Buenos Aires and Germany, and has followed investigations to Asia, South America and the Middle East.