(a) Opinion and reputation evidence of character. The credibility of a witness may be attacked or supported by evidence in the form of opinion or reputation, but subject to these limitations: (1) the evidence may refer only to character for truthfulness or untruthfulness, and (2) evidence of truthful character is admissible only after the character of the witness for truthfulness has been attacked by opinion or reputation evidence or otherwise.

(b) Specific instances of conduct. Specific instances of the conduct of a witness, for the purpose of attacking or supporting the witness' credibility, other than conviction of crime as provided in Rule 609, may not be proved by extrinsic evidence. They may, however, in the discretion of the court, if probative of truthfulness or untruthfulness, be inquired into on cross-examination of the witness: (1) concerning the witness' character for truthfulness or untruthfulness, or (2) concerning the character for truthfulness or untruthfulness of another witness as to which character the witness being cross-examined has testified. No specific instance of conduct of a witness may be the subject of inquiry under this provision unless the cross-examiner has a factual basis for the subject matter of his inquiry.