The object of a police lineup is the correct identification of a suspect by an eyewitness.
A Formal Lineup is done at the station. The suspect and 4-6 “fillers” are presented to the witness standing side by side in a well-it room. The witness is in an adjacent room looking at the lineup through a one-way mirror.
More commonly, a Photographic Lineup is used. Here, the suspect’s photograph is mixed among 5 or 6 “filler” photographs. The photos may be shown to the witness one at a time (sequential photo lineup), or all at once on a single large page (simultaneous photo lineup).
In either method, the following caveats are important:
The administrator must refrain from leading the witness. For example, if the witness picks a filler as the suspect, the administrator must not say something to the effect of “take your time”, or “be sure to look at all of the pictures before you chose”, and so forth.
Be sure that the fillers resemble the suspect, and are similarly attired.
Be aware of weapon distraction. If the crime involved the use of a weapon, the witness may focus on the weapon and have little or no idea what the suspect looked like.
Be aware that studies have shown that the most reliable lineup (either photographic or formal, live lineup) is done in a double blind fashion. This means that the administrator does not know who the suspect is. The detective on the case may participate in choosing the fillers, but the lineup itself should be conducted by another detective who is unaware of who the suspect is.
Finally, be aware that the most reliable photographic lineup is the double-blind, sequential photo lineup.