When a TV show stays on the air for two decades like "Law and Order" did during its run on NBC, there is a good chance it is going to feature plenty of guest stars who go on to bigger and better things in their own movies and TV shows. These now famous faces were intimately involved in "Law and Order" cases throughout the years before becoming household names.

Jennifer Garner 

Before carving out an identity as superspy Syndey Bristow in "Alias," Garner appeared in a 1996 episode of "Law and Order" as the college-aged mistress of a police detective. Garner later launched into movie career after "Alias" ended, although she has scaled back her career while raising a family with actor husband Ben Affleck.

Philip Seymour Hoffman

Few actors vanish into a character as effectively as Hoffman. He can do everything from effeminate author Truman Capote in "Capote" to a ruthless supervillain in "Mission Impossible III." Hoffman appeared on "Law and Order" in 1991. In keeping with his penchant for playing dark characters, he portrays a drug dealer who is accused of raping a celebrity journalist.

Ellen Pompeo

Before Pompeo gained fame as a doctor saving lives and searching for love on "Grey's Anatomy," she cut her teeth taking lives on "Law and Order." Pompeo appeared in two different episodes in 1996 and 2000. In the one episode, her character is complicit in murdering her mother. In the other episode, she is involved in her sister's death.

Julianna Marguilies

Long before he became her adulterous husband in "The Good Wife," Margulies shared the screen with Chris Noth as a naval officer in a 1991 "Law and Order" episode centered on the murder of a female naval officer at a party in a Manhattan hotel. Margulies hit it big a few years later when she played a nurse, Carol Hathaway, on the first few seasons of "ER."

Samuel L. Jackson

Is there a movie or TV show out there these days that Jackson hasn't appeared in? From brandishing a lightsaber to battling snakes on a plane, Jackson has done everything. That includes a guest stint on "Law and Order" in 1991, on the same episode that also featured Phillp Seymour Hoffman. Jackson would break out as a movie star three years later with his turn as a hit man in "Pulp Fiction."