A criminal defendant has a federal constitutional right, grounded principally in the Fifth Amendment Privilege against Self-Incrimination, to testify at his or her trial in his or her own defense if the defendant has any relevant testimony to give about his or her guilt or innocence.

United States v. Ledford, 443 F.3d 702, 710 (10th Cir. 2005) (upholding trial court's admission of the testimony of Deputy Brodheim about Ms. Carey's statement [on being scared since defendant had told her he would kill....A statement falls within the hearsay exception created by Federal Rule of Evidence 803(2) or a comparable State rule for excited utterances, only if the declarant (the maker of the statement) was under the stress of the startling event or condition when he or she made the statement that relates to that startling event or condition.

Perrin v. Anderson, 784 F.2d 1040, 1044 (10th Cir. 1986) (holding, in an action against patrolmen for deprivation of decedent's civil rights, that evidence of decedent's previous violent encounters with police was erroneously admitted where it was offered to show that decedent was the aggressor).When an exception to the character evidence prohibition makes evidence of a criminal defedant's character admissible, the testimony of a witness on direct examination about the defendant's character must take the form of reputation or opinion and it cannot consist of specific instances of the defendant's conduct.

United States v. Magallanez, 408 F.3d 672, 680 (10th Cir. 2005) (upholding the trial court's admission of testimony of a government rebuttal witness; held, witness' testimony was not character evidence, but was properly introduced as rebuttal evidence which is not violative of either Rule 608 or Rule 404).If a criminal defendant offers evidence of his or her character circumstantially - to show he or she did not commit the crime charged -- the prosecution can offer rebutting evidence of defendant's character.