Campbell is due to appear in court Thursday. But Taylor's attorney says the prosecution has failed to disclose exactly what she is expected to say, preventing him from preparing his cross examination.
Prosecutors say Campbell has refused to talk to them, and is being forced to appear under a subpoena.
They intend to ask her about information that she received diamonds from Taylor after a dinner party in South Africa in 1997. But they have little other information.
Taylor is accused of receiving rough diamonds in exchange for his support for rebels in Sierra Leone during that country's 11-year civil war.