An amicus brief assists the court with a perspective the parties cannot supply. Briefs that merely repeat a party’s arguments are unhelpful and are recognised as such.
Leave and consent. Consent of the parties, or leave of court, is required in most systems, with disclosure of who authored the brief and who funded it.
Timing. Filed within a short period after the supported party’s brief.
What makes an amicus brief valuable. Industry or practical consequences the parties are not positioned to describe; empirical data; the experience of non-parties who will be affected; the law of other jurisdictions; and historical or technical background.
What makes one worthless. Restating the party’s argument; general assertions of importance; and advocacy that reads as a second brief for a party.
Coordination. Where several amici support the same side, coordination avoids duplication and allows each to take a distinct angle. Courts notice when ten briefs make the same three points.
Length and focus. Shorter is better. A brief making one point well is read; one covering everything is skimmed.
At the certiorari stage amicus briefs serve a different function — showing that the question recurs and matters beyond the parties — and are often more influential there than on the merits.