Fred Thompson portrayed D.A. Arthur Branch in the main Law & Order series as well as some of the spin-off series (namely, Conviction, L&O SVU, L&O Trial by Jury and L&O CI).
Before his acting days he worked as an attorney. According to his Wikipedia page:
Thompson was admitted to the state bar of Tennessee in 1967. At that time, he shortened his first name from Freddie to Fred. He worked as an assistant U.S. attorney from 1969 to 1972, successfully prosecuting bank robberies and other cases. Thompson was the campaign manager for Republican U.S. Senator Howard Baker's re-election campaign in 1972, and was minority counsel to the Senate Watergate Committee in its investigation of the Watergate scandal (1973–1974).
In the 1980s, Thompson worked as an attorney, with law offices in Nashville and Washington, DC, handling personal injury claims and defending people accused of white collar crimes. He also accepted appointments as special counsel to the Senate Foreign Relations Committee (1980–1981), special counsel to the Senate Intelligence Committee (1982), and member of the Appellate Court Nominating Commission for the State of Tennessee (1985–1987).
His clients included a German mining group and Japan's Toyota Motors Corporation. Thompson served on various corporate boards. He also did legal work and served on the board of directors for engineering firm Stone & Webster.
Wikipedia continues to describe his exposing a cash-for-clemency scheme that removed Governor Blanton of Tennessee from office, his time as a US senator, and even his run for president in 2008.
His first film, Marie (1985) in which he stars as himself in a thriller about the aforementioned Tennessee governor case, introduced him to acting.
Sig Libowitz portrayed defense attorney Stan Shatenstein in L&O CI and in the main L&O series. His last appearance in the L&O franchise was in 2005. Wikipedia states that he did graduate law school in 2007 so he was likely to be a law student at least on his last appearance in the series.