State witness competence rules apply in federal civil actions with respect to elements of claims or defenses for which state substantive law supplies the rule of decision.

Cf. Rock v. Arkansas, 483 U.S. 44, pin? (1987) (held, trial court violated homicide defendant's constitutional rights by barring her from testifying about the circumstances of the killing on the ground that....The Compulsory Process Clause of the Sixth Amendment to the U.S. Constitution grants a criminal defendant the right to introduce exculpatory evidence that is probative of and relevant to a material issue.

Fed. R. Evid. 414 ("In a criminal case in which the defendant is accused of an offense of child molestation, evidence of the defendant's commission of another offense or offenses of child molestation is admissible, and may be considered for its bearing on any matter to which it is relevant.").Federal Rule of Evidence 414 and comparable rules in a few States create an exception to the prohibition against "circumstantial" use of character -- the prohibition against the use of character to show conduct -- by providing that in a criminal trial for child molestation or child sexual abuse, evidence of the defendant's commission of another offense or offenses of child molestation or child sexual abuse is relevant and admissible.

United States v. Abel, 469 U.S. 45, pin? (1984).Extrinsic evidence may be used to to impeach a witness by showing the bias or interest of the witness; i.e., the party seeking to impeach a witness by showing bias or interest is not limited to the testimony of the target witness and may use evidence other than the testimony of the target witness to show the bias or interest of the target witness.