Trial judges are entitled to question witnesses. Indeed, sometimes they are duty-bound by the interests of justice to ask questions. They are not required to remain silent, passive observers of trials. There are, however, limits on their ability to intervene in the examination of witnesses. Their interventions must never compromise the overall appearance of fairness in the trial proceedings.
The decision in
R. v. Lahouri, 2013 ONSC 2085 sets out a series of guidelines applicable to a judge's questions at trial:
(1) Refrain From Usurping the Role of Counsel: As a general rule, a trial judge should endeavour to confine him or herself as much as possible to his or her own responsibilities and leave counsel to perform their functions in the litigation. The criminal trial process is an adversarial one, and should not become an inquisitorial investigation by the trial judge. Accordingly, the examination of witnesses must remain, for the most part, the responsibility of counsel, and a trial judge must be careful not to effectively usurp that role. See: R. v. Valley, at pp. 230-231; R. v. Torbiak and Campbell (1974), 18 C.C.C. (2d) 229 (Ont.C.A.) at pp. 230-231; R. v. Stucky, at para. 63; Chippewas of Mnjikaning First Nation v. Chiefs of Ontario, 2010 ONCA 47 (CanLII), at para. 237.
(2) The Right to Pose Questions: A trial judge is justified in: (a) posing questions to a witness to clear up ambiguities in their evidence; (b) calling a witness to order and focusing him or her on the true matters in issue; (c) exploring some issue on which the witness's evidence has been left vague and uncertain; or (d) putting questions which should have been asked by counsel in order to elicit evidence on some relevant issue. See: R. v. Brouillard, at p. 47; R. v. Valley, at p. 230; R. v. Bateman (1946), 31 Cr.App.R. 106, at pp. 110-112; R. v. Stucky, at para. 64; R. v. Watson(2004), 2004 CanLII 45443 (ON CA), 191 C.C.C. (3d) 144 (Ont.C.A.) at para. 10. The right to pose questions that should have been asked by counsel is not, however, an open-ended invitation to the trial judge to usurp the role of the counsel. See: R. v. Valley, at p. 230; R. v. Stucky, at para. 65.
(3) The Timing of Questions: Generally speaking, substantive questions going beyond the clarification of an answer should be posed by a trial judge only after counsel has completed his or her examination of the witness. Otherwise, the trial judge risks interfering with the prepared organization and flow of the testimony. See: R. v. Valley, at p. 230; R. v. Stucky, at para. 64; R. v. Torbiak and Campbell, at p. 231; Chippewas of Mnjikaning First Nation v. Chiefs of Ontario, at para. 239.
(4) No Cross-Examination: The right of a trial judge to question any witness does not entitle a trial judge to cross-examine witnesses. Accordingly, a trial judge should not, even temporarily, abandon his or her position of neutrality and become the cross-examiner, especially in cases where he or she is the ultimate fact-finder. See: R. v. W.(A.) (1994), 1994 CanLII 218 (ON CA), 94 C.C.C. (3d) 441 (Ont.C.A.) at para. 38-42; Reversed: 1995 CanLII 83 (SCC),  4 S.C.R. 51; R. v. Valley, at p. 230.
(5) Permitting the Accused to Give His or Her Evidence: The duty on a trial judge to exercise restraint and remain neutral is especially critical in cases where the accused takes the stand to give evidence. A trial judge must allow the accused to give his or her evidence freely and must not intervene in the examination of the accused so as to effectively preclude the accused from telling his or her story in his or her own way. See: R. v. Brouillard, at p. 48; R. v. Valley, at pp. 231-232; R. v. Stucky, at para. 66, 71; R. v. Russell, 2011 BCCA 113 (CanLII); 302 B.C.A.C. 208, at para. 16-19; R. v. Adano,  O.J. No. 1995 (S.C.J.) at para. 21-24, 36.
(6) Remaining Neutral: A trial judge must not question an accused or a defence witness to such an extent or in such a manner that conveys the impression that the trial judge has placed the authority of his or her judicial office on the side of one of the parties in the litigation. See: R. v. Valley, at pp. 231-232; R. v. Stucky, at para. 71; R. v. Augello,  3 C.C.C. 191 (Ont.C.A.).
(7) Refrain From Interfering With the Conduct of the Defence: A trial judge must not intervene in the trial to such an extent that it effectively renders it impossible for defence counsel to perform his or her duty in advancing the defence on behalf of the accused. For example, the trial judge should not divert counsel from his or her chosen topic of examination, or interfere with the sequence and dynamic flow of cross-examination so as to prevent the proper testing of the testimony. See: R. v. Valley, at pp. 231-232; R. v. Stucky, at para. 71; R. v. Matthews(1983), 78 Cr.App.R. 23, at p. 31; R. v. Kitaitchik (2002), 2002 CanLII 45000 (ON CA), 166 C.C.C. (3d) 14 (Ont.C.A.) at para. 13; R. v. Watson, at para. 14.