A witness is to testify as to facts - not to explain their theory of a case.
R. v. Dhaliwal, 2016 ONCA 652:
 This line of questioning was improper. It put an onus on the appellant to explain the allegations against him: R. v. S.(W.) (1994), 18 O.R. (3d) 509, 90 C.C.C. (3d) 242 (C.A.) at 252; R. v. Vandenberghe (1995), 96 C.C.C. (3d) 371 (C.A.) at 373.
 The point was made by the Alberta Court of Appeal in R. v. Kusk, 1999 ABCA 49, 132 C.C.C. (3d) 559, at p. 564:
This mischievous cross-examination wrongly suggests that the witness is advocating a certain view, indeed advocating corollaries of that view. See R. v. Baldwin,  All E.R. Rep. 402, 18 Cr. App. R. 175 (Eng. C.A.), 178-79 (C.C.A.). That runs together the three roles of witness, accused, and defence counsel. When the accused testifies, he is a witness, not an advocate. The accused may try to cooperate in answering the forbidden question, or he may vaguely feel that something is wrong with it, but not one lawyer in 10,000, let alone a lay person, could say on the spot what that wrong thing was: R. v. Baldwin. Here the accused vaguely saw the point, and his answer (quoted above) stumbled toward what the Court of Criminal Appeal said in 1925. Yet counsel and the trial judge missed the valid point which he was groping to express, albeit incompletely. And the jury may have felt, as the Court of Criminal Appeal points out, that inability to answer substantively indicated concealment.
 Asking the appellant, in front of the jury to provide his "theory" of the case or to explain the evidence against him undermined the presumption of innocence. Permitting the Crown to ask the question, and requiring the appellant to answer it, could only have led the jury to believe that he had some obligation to provide a "theory". The line of questioning should not have been permitted.
Of the Law Societies of Upper Canada and Nunavut