The focus of a criminal trial is on whether the accused committed the offence charged.
As a general rule, a Crown is not permitted to elicit evidence unrelated to the charge that tends to show that an accused is a person of bad character.
Such evidence is problematic because it distracts the jury or judge from the specific allegation before the Court.
It is also dangerous because the accused may be wrongfully convicted because of the type of person that the jury or judge perceives him to be rather than on the quality of the evidence pertaining to the charge. The decision-making process is compromised when a jury or judge reasons that the accused is generally a 'bad guy’ who would do this ‘type of thing’ and uses that as evidence of guilt at trial.
Defence counsel must be ready to object when a Crown cross-examines an accused on discreditable conduct that is unrelated to the specific allegation before the Court. The default position is that such questions are impermissible.
1) When an accused chooses to testify, the Crown is permitted to cross-examine the accused on his criminal record of conviction
Subject to a successful Corbett application, the state is permitted to cross-examine an accused on the 'bare bones' of his criminal record of conviction for the limited purpose of challenging his credibility (see Canada Evidence Act s. 12 and R. v. Corbett,  1 S.C.R. 670).
2) The accused may open the door for cross-examination on bad character evidence by tendering evidence of good character
If an accused tenders evidence that he is not the type of person to commit the alleged offence, the accused's character becomes a live issue for cross-examination.
Even when the accused puts his character in issue, the Defence must ensure that the Crown's rebuttal evidence does not exceed permissible limits so as to render the trial unfair.
R. v. Rose,  O.J. No. 1150 (C.A.) at para. 26:
…Crown counsel transgressed the limits of relevance in questioning the appellant generally on his lifestyle, from the manner of his dress to the fulfillment of his fiscal responsibilities. The appellant was not on trial for his general lifestyle and it was unfair to place him in a position where he had to defend against vague and irrelevant suggestions of improper conduct.
R. v. Bouhsass,  O.J. No. 4177 (C.A.) at para. 11:
…The language used … measured the appellant against a severe moralistic standard. The appellant was attacked for his lifestyle, including his relationship with women in general, his sexual activities, his supposed heroin addiction and his "thievery". While some of these matters (the thefts and the heroin addiction) were held by the trial judge to have probative value, as indicated, the Crown's questions focused largely on the prejudicial content of the evidence, rather than its probative content.
R. v. R. (A.J.),  O.J. No. 2309 (C.A.), beginning at para. 34:
There are numerous instances in the cross-examination when the questions went beyond the bounds of relevancy and legitimate credibility impeachment and became an attempt to highlight the appellant's bad character and deviant lifestyle. The appellant was cross-examined about whether he had filed income tax returns. He was also questioned about the criminal records of his associates and about his attitudes, as "a former drug dealer", to T.'s use of prescription pills. Still later, Crown counsel asked the appellant about his respect "for the law and court orders". Crown counsel also cross-examined the appellant to show that he was sexually promiscuous and had no sense of responsibility to any of the women with whom he had been involved during his lifetime. At another point in the cross-examination, she referred to the appellant as "a jailhouse lawyer".
The appellant was also cross-examined about the paternity of C., T.'s young son. He denied that he was the father. Crown counsel then gave evidence to the effect that C. could not be adopted because it was believed that he was the product of an incestuous relationship. She then asked the appellant why he did not submit to a blood test so that the question of C.'s paternity could be cleared up and C. could become eligible for adoption. None of this had anything to do with the allegations against the appellant and could only serve to inflame the jury further against the appellant….