Fed. R. Evid. 405

(a) By Reputation or Opinion.

When evidence of a person’s character or character trait is admissible, it may be proved by testimony about the person’s reputation or by testimony in the form of an opinion. On cross-examination of the character witness, the court may allow an inquiry into relevant specific instances of the person’s conduct.

(b) By Specific Instances of Conduct.

When a person’s character or character trait is an essential element of a charge, claim, or defense, the character or trait may also be proved by relevant specific instances of the person’s conduct.