Anyway, noting that the cast of the show was regularly composed of recognized actors, I will focus on actors who appeared in an 3 episodes at most, as a perp, as a witness, as a victim... They typically fall into two groups: actors on L&O before their break somewhere else, and actors on L&O after making it somewhere else. People love to slum and do L&O, I would if I was an actor, but I would do everything coming my way. Like Samuel L. Jackson. Breaks and latest are subjective, and I can't remember all of the episodes listed, but will revise them if I see them.
Some of my favorites:
Chevy Chase
1 episode, 1 characterBreak: "Saturday Night Live" (1975)
Latest: Probably this, or Snow Day (2000).
Episode: "In Vino Veritas" (2006) A washed-up, anti-Semitic actor is arrested with blood on his clothes. Detectives later discover that a Jewish television producer he has connections to has been murdered. As I mentioned earlier, this is Chevy Chase as the Mel Gibson DUI episode times a million. Except Mel Gibson wasn't washed up at the time of his arrest. A must-see for Chevy Chase fans, no comedy here, just awkward drama.
Julia Roberts
1 episode, 1 characterBreak: Steel Magnolias (1989) was good, but Pretty Woman (1990) made her.
Latest: Charlie Wilson's War (2007) I didn't see it.
Episode: "Empire" (1999) A businessman planning to build a new football stadium in New York City is on trial for murder. But the prosecution's star witness, a professional fundraiser who was having an affair with the victim, may have her own sinister motives. Julia Roberts makes a major play for my heart as she slums it to play the sinister fundraiser in this episode of L&O. I've mulled it over and cannot find any good reason for her to do this except that she's a fan, which would be awesome.
Brad Renfro/Ethan Embry
Breaks: The Client (1994) for Brad, Dutch (1991) or Empire Records (1995) for Ethan.
Latest: Renfro had a few good roles, but ultimately succumbed to his drug addiction in January. Ethan is in Eagle Eye (2008) which I hear is terrible.
Episode: CI: "Watch" (2006) Logan and Barek pursue the trail of a serial killer when a body drops from the sky into the ocean near J.F.K. I forget how this episode goes, but they are both suspects and both look terrible. I remember liking this episode.
Tom Berenger
1 episode/1 character
Break: Looking for Mr. Goodbar (1977)
Latest: Training Day (2001)
Episodes:
"Panic" (2000) Detectives suspect that a best-selling novelist was shot because she was having an affair with a married FBI agent who was helping her with research. It turns out that they have the right motive, but the wrong suspect. She was cheating with his wife! Booyah Tom Berenger!
Stephen Colbert
1 episode (CI)/1 character
Break: "The Daily Show" (1997-2005)
Latest: "The Colbert Report", "Strangers with Candy", "Harvey Birdman"
Episode:
CI: "The Saint" (2004) The detectives try to determine why a social worker was killed with a lye bomb, and whether it has anything to do with a Catholic saint. And he plays the document-verifier/master forger/mama's boy at the bottom of all of it! Bizarre.
Amy Sedaris
1 episode (SVU)/1 character
Break: "Strangers with Candy" (1999-2000)
Latest: Strangers with Candy (2005)
Episode:
SVU: "Head" (2004) The fifth season ends with a bizarre pedophilia story involving a 12-year-old student, his alcoholic mother, and his principal. She plays the bit part of the sister of the guy whose toilet cam witnessed the crime. She worked at a spy store and gave him the camera, he was kind of stupid I think.
Hayden Panettiere
2 episodes (SVU)/2 characters
Break: I'm inclined to say "Heroes", but she was in "One Life to Live" (1994-1997) and "The Guiding Light" (1996-2000).
Latest: "Heroes"
Episodes:
SVU: "Abuse" (2000) Benson and Stabler suspect parental neglect in the death of the son of two famous singers, The detectives fear for their surviving daughter. She plays the daughter who keeps trying to move in with Mariska Hargitay.
SVU: "Hooked" (2005) When the body of a teenager is found with her older cousin's I.D., the detectives learn the teen had been part of a group who had 'hooked up' online, and the victim was the object of an older man's obsession, who also turns up dead. I remember a part where the cops bust her fellating some guy in the back of a high-end boutique for a purse or something.
Rob McElhenney
1 episode/1 character
Break/Latest: "It's Always Sunny in Philadelphia" (2005-2008)
Episode:
"Thrill" (1997) Two punks kill a random victim for the thrill of it. But the prosecution faces the difficult task of convicting two people of the same crime. He plays the one who gets off, if that's what happened. He's incredibly young-looking in this appearance.
Michael McKean
2 episodes, 2 characters
Break: "Laverne & Shirley" (1976-83)
Latest: Probably For Your Consideration (2006), which I haven't seen.
Episodes:
"Mega" (2000) Six people die in a helicopter explosion. Evidence points to a millionaire con-man who provides self-actualization seminars for his wealthy clients. Regular L&O writer Lynn Mamet wrote this script with McKean and wife Annette O'Toole in mind. He, of course, plays the con man.
"Called Home" (2008) In an investigation of a suspected assisted suicide, the detectives uncover the involvement of a recently-released-from-jail Dr. Kevorkian-stlyed character, his daughter, and a TV talk show host. He plays the talk show host, conniving, ruthless... this episode came on while I was writing this.
Larry Miller
3 episodes, 1 character and himself
Break: Longtime stand-up comedian, caught attention with Pretty Woman (1990). I liked him in Necessary Roughness and Suburban Commando (both 1991).
Latest: It's probably not the most recent, but I loved his bit part in Best in Show (2000).
Miller gets the rare treat of reprising his role as a suspect, jerk comedian/comedy club/restaurant owner Michael Dobson. One of my favorite guest stars.
Episodes:
"Coma" (1994) When a woman is shot to death inside her car, Briscoe and Logan suspect her husband is the killer.
"Encore" (1996) A businessman most people suspect got away with conspiring to murder his first wife is now accused of conspiring to kill his second wife. Both cases share eerie similarities.
"Smoke" (2003) The child of a popular comic dies after he is reportedly thrown out of a window during a fire. The investigation also uncovers allegations that the comic molested an 11-year-old boy years earlier. Miller plays himself as a material witness in this L&O take on Michael Jackson. Instead of a singer, he's a comic, just like when they took on O.J. Simpson and made him a baseball player.
John Heard
2 episodes, 2 characters (plus 1 episode CI, 1 episode SVU, for 4 different characters!)Break: He was in C.H.U.D (1984), and things before that, but I didn't know him until Big (1998).
Latest: Honestly, I don't know. He was in "Prison Break" for about 10 episodes.
Episodes:
"Doubles" (1994) No IMDb synopsis. :(
"DNR" (1999) Judge Denise Grobman is shot and seriously wounded when a man steals her car. Yet when investigation leads to a hired hit ordered by her husband, she vehemently refuses to believe in his guilt. Ooh, he's the bad guy. Good episode for him.
CI: "The Pardoner's Tale" (2001) When an investigative reporter and his fiancée are gunned down, the detectives follow a trail to organized crime and a bribe that was paid at the state's highest level. I have no idea who he was.
SVU: "Disappearing Acts" (2002) A rape investigation finds the detectives dealing with the FBI, as the suspect (and his father) are in witness protection, following his previous rape of a woman with ties to the Russian mafia. The FBI asks the detectives to back off, but in the the end, they both die and everybody learns a lesson in futility. This episode also features Pam Grier as the US attorney.
William H. Macy
2 episodes, 2 charactersBreak: It's hard for me to place him, as he was still going by W.H. Macy for his first L&O appearance, and he didn't get enough attention until Fargo (1996), though he had plenty of good roles before then.
Latest: The last thing I saw him in was Thank You for Smoking (2005).
Episodes:
"Everybody's Favorite Bagman" (1990) The assault of a city councilman uncovers a scandal involving organized crime, elected city officials, and a deputy police commissioner. He plays the assistant US attorney. Typical.
"Sisters of Mercy" (1992) A young teenage runaway is arrested after discharging a gun in a diner. Cerreta and Logan learn that she had been raped at the halfway house she had been staying in, and the investigation leads to a free thinking nun and the caretaker of the halfway house. I know I've seen him in one of these, but I don't remember. Wouldn't that be weird if he was the halfway house caretaker/rapist?
Samuel L. Jackson/Philip Seymour Hoffman
1 episode/2 characters
Breaks: Hard to say, Jackson had small roles in a number of Spike Lee movies before this episode, so this isn't exactly beforeh is break. For Hoffman, either Leap of Faith or Scent of a Woman (both 1992). This is his first listed acting credit.
Latest: What isn't Sam Jackson in lately. Lakeview Terrace (2008). For Hoffman, probably Charlie Wilson's War (2007).
Episode:
"The Violence of Summer" (1991) Stone temporarily dismisses rape charges against three defendants because the victim's testimony has too many holes. Greevey and Logan's subsequent reinvestigation uncovers a possible fourth assailant. Damn, I haven't seen this gem of an episode either.
Others: Wikipedia's exhaustive list
