Federal Rules of Evidence - Rule 405. Methods of Proving Character
(a) Reputation or opinion.
In all cases in which evidence of character or a trait of character of a person is admissible, proof may be made by testimony as to reputation or by testimony in the form of an opinion. On cross-examination, inquiry is allowable into relevant specific instances of conduct.
(b) Specific instances of conduct.
In cases in which character or a trait of character of a person is an essential element of a charge, claim, or defense, proof may also be made of specific instances of that person's conduct.