Generally, live lineups don't seem to be more likely to produce accurate identifications than showups and photo identifications. Experimental studies show that witnesses who have a chance to view the actual perpetrator aren't more likely to make an accurate identification when they see the suspect in a lineup rather than in another type of pretrial identification procedure.
Police officers and often a prosecutor attend lineups. A defense attorney may be present as well, because a suspect who has been formally charged with a crime has a right to be represented by a lawyer at a lineup. (Kirby v. Illinois, 406 U.S. 682 (1972).) In large cities, public defender offices may have an attorney available to attend a lineup 24/7. The defense lawyer may also bring an investigator, a paralegal, a law clerk, or another observer to act as a witness in a later court hearing in case the lineup procedures are unfair to the defendant. To avoid having a defense lawyer present, an officer may try to convince a suspect to participate in a lineup voluntarily before the filing of charges.
The police can typically force someone who has been arrested to participate in a lineup. Judges don't consider this a violation of the Fifth Amendment privilege against self-incrimination because, in a lineup, suspects don't provide "testimony." (United States v. Wade, 388 U.S. 218 (1967).)
As a condition of granting bail or OR release, a judge may require a suspect to participate in a lineup. However, being released from jail may reduce a suspect's chance of having to participate in a lineup because of the logistics of arranging it.
Unless they have a court order, the police cannot compel suspects who have not been arrested to participate in a lineup. The police may ask such suspects for voluntary participation, arguing that "this is our chance to clear you." But even those who are confident of their innocence should think carefully—and talk to a lawyer—before agreeing to participate in a lineup voluntarily. After all, witnesses make mistakes. (See The Psychology of Eyewitness Identification.)
Laws in many states give suspects the right to demand a lineup. But suspects should think carefully—and, again, talk to an attorney—before doing so. The advantage of participating in a lineup is that eyewitnesses' failure to make a positive identification may cause the police to focus their investigation elsewhere. The disadvantage of mistaken identification is the flip side. (The defendant might also accidentally provide fodder for the prosecution in the course of the events surrounding the lineup—for example, by making a comment that turns out to be incriminating.)
One-way mirrors or bright lights typically make it impossible for suspects to see witnesses. Even if contact is possible, suspects definitely shouldn't try to talk to or otherwise interact with witnesses. Even a plaintive "Tell them I'm innocent" may lead to additional scrutiny from the witness and officers. The police may even construe a suspect's attempt to talk to a witness as intimidation and cause the suspect to be charged with a separate crime!
The police can generally dictate what participants wear and say during a lineup. Dressing the lineup participants as the culprit was dressed, and having them speak words that the culprit used, can increase the likelihood that an eyewitness's identification (or failure to identify) is accurate. Of course, for the lineup to be fair, conditions must be the same for all participants.
Example: Ann Ekdote is arrested for burglarizing a home. Wilma, the next-door neighbor, tells the police that the burglar was a woman who wore large sunglasses, carried a big shopping bag, and yelled "It's all mine!" while running out of the house. The police arrest Ann and ask Wilma to view her in a lineup. The police can dress Ann in large sunglasses and have her carry a big shopping bag if the items match Wilma's description, as long as all of the lineup participants are displayed to Wilma in the same way. They can also require each participant to yell, "It's all mine!" Because the other participants will do as the police request, Ann is likely to draw more attention to herself by refusing to repeat the words (or by whispering them). Moreover, the prosecution can use Ann's refusal as further evidence of her guilt at trial.
Always enforce your right to counsel when facing a lineup request—or any other entreaty—from police. You should consult a knowledgeable lawyer before making any critical decisions in a criminal case. Your attorney can advise you of the nuances of the law, including how they apply to the facts of your case.