735 ILCS 5/8-101: Interested witness.

No person shall be disqualified as a witness in any action or proceeding, except as hereinafter stated, by reason of his or her interest in the event thereof, as a party or otherwise, or by reason of his or her conviction of any crime; but such interest or conviction may be shown for the purpose of affecting the credibility of such witness; and the fact of such conviction may be proven like any fact not of record, either by the witness himself or herself (who shall be compelled to testify thereto) or by any other witness cognizant of such conviction, as impeaching testimony, or by any other competent evidence.