What is the general principle of a jury trial?

The general principle, frequently quoted, was stated by Taschereau J. in Azoulay v. The Queen 1952 CanLII 4 (SCC), [1952] 2 S.C.R. 495 at 497-498: The rule which has been laid down, and consistently followed is that in a jury trial, the presiding judge must, except in rare cases where it would be needless to do so, review the substantial parts of the evidence, and give the jury the theory of the defence, so that they may appreciate the value and effect of that evidence, and how the law is to be applied to the facts as they find them.

In support of this proposition, Taschereau J. cited Spencer v. Alaska Packers (1905) 1904 CanLII 23 (SCC), 35 S.C.R. 362. In that case, Nesbitt J. expressed the principle this way at 371: I do not think the judge is bound to comment upon evidence in the sense of reviewing what the several witnesses have sworn to, or to point out for consideration of the jury anything which may strike him as throwing light upon the credibility of the story, but I think he is bound to direct the jury as to the law and to direct their attention how that law is to be applied to the facts before them according as they find them.