What is the duty of a police officer to intervene when fellow officers violate the constitutional rights of a suspect or other citizen?

"[O]fficers have a duty to intercede when their fellow officers violate the constitutional rights of a suspect or other citizen." United States v. Koon, 34 F.3d 1416, 1447 n.25 (9th Cir. 1994), rev'd in part on other grounds, 518 U.S. 81 (1996) (citations omitted). "In these cases, the constitutional right violated by the passive defendant is analytically the same as the right violated by the person who strikes the blows." Id. "Importantly, ... officers can be held liable for failing to intercede only if they had an opportunity to intercede." Cunningham v. Gates, 229 F.3d 1271, 1289 (9th Cir. 2000), as amended (Oct. 31, 2000) (citation omitted).