A Voir Dire is a hearing to determine a question of law, including the admissibility of evidence. It is typically held during a trial but is considered a separate hearing from the trial itself. It is known as a "trial within a trial" and designed to determine an issue separate from the trial on matters of procedure or admissibility of evidence.
R v Sadikov, 2014 ONCA 72 (CanLII) at para 30 (A voir dire is to "determine the admissibility of evidence proposed for admission by a party to a criminal proceeding")
The procedure in holding a voir dire is at the discretion of the judge based on the issue and nature of the "means of proof available".
Failure to Follow Procedure
The failure of holding a voir dire to determine if potentially inadmissible evidence should be heard will not not always be fatal to the trial. Where the evidence is still tested and there is no prejudice to the accused to may still be valid. The court must consider whether the process was followed that served the same purpose as the voir dire.
There seems to be some ability to make a motion for "non-suit or directed verdict" motion by the responding party to a voir dire application.
Timing of voir dire
It has been recommended that judges ruling on voir dires "should resist making final rulings until such time as they required to do so". As trial progresses many issues may resolve themselves. However, where the "proposed evidence is likely to have a significant impact on the outcome of the trial" then it should be dealt with early. 
Multiple Void Dies
Each evidentiary issue should be treated as a separate voir dire and their evidence cannot be joined without consent. That being said, particular pieces of evidence may raise multiple issues of admissibility and are usually treated together. Caution must be taken by the judge to ensure their ruling reflect "an informed understanding" of the governing law.
- R v Sadikov, 2014 ONCA 72 (CanLII) at para 32
- R v D.A.R., 2012 NSCA 31 (CanLII)
- R v Gartland, 1981 CarswellOnt 1845, 7 W.C.B. 110 (*no link) at para 26 (the accused person must be afforded "every essential procedural step and safeguard available to him on his trial on the merits of the substantive offence, in so far as it can be applicable...") cited also in R v T.(B.) 2012 NSPC 59 (CanLII)
- R v Viszlai, 2012 BCCA 442 (CanLII) at paras 69 to 72
- Viszlai, ibid.
- R v Harris, 1997 CanLII 6317 (ON CA), at para 38
- Sadikov at para 31
- Sadikov at para 33
Types of Voir Dires
Applying Rules Evidence
The judge must determine whether the "conditions precedent" to the admission of evidence have been met.
Sadikov at para 30
Constitutional Challenges to the Admissibility of Evidence
The process of challenging the constitutionality of the admissibility of evidence requires first an inquiry into the constitutionality of the state's conduct and then second, should a finding of unconstitutionality is found, an inquiry into the "admissibility of the evidence obtained by the infringement".
Provincial Rules of Court may also provide guidance on procedure.
- R v Sadikov, 2014 ONCA 72 (CanLII) at para 35
- e.g. Ontario: Rule 31 of the Criminal Proceedings Rules
Challenging Warrant Validity
Procedurally, a voir dire on the for the validity of a warrant should proceed as follows:
- (a) The trial judge should determine whether a voir dire is necessary and, if so, whether the calling of evidence should be permitted;
- (b) If the judge accedes to the request to hold a voir dire and the accused wishes to cross-examine the informant, then the accused must obtain leave of the judge to do so. If the judge grants leave, then he or she can limit the scope of the cross-examination;
- (c) Cross-examination should proceed to the extent permitted by the order granting leave;
- (d) Re-examination, if any, should follow the cross-examination; and
- (e) The trial judge should determine whether the record as amplified on the review could support the issuance of the warrant.
R v Wilson, 2011 BCCA 252 (CanLII) at para 69
A voir dire cannot be waived by mere silence of counsel.
Unequivocal admission of the issue by an opposing party should generally be sufficient to forgo a voir dire. The judge should be be satisfied that counsel understood the matter and has made an informed decision.
Voir Dire Evidence
Charter applications require a factual record. They cannot be argued in a vacuum.
Affidavits that are based on hearsay should be given little weight and generally should not be considered admissible as evidence on a Charter application.
Since evidence in a voir dire is separate and apart from evidence in the trial proper, each exhibit should be marked to be distinguished from the trial such as "V.D. Exhibit 1, etc".
See MacKay v Manitoba, 1989 CanLII 26 (SCC),  2 SCR 357
Danson v Ontario, 1990 CanLII 93 (SCC),  2 SCR 1086
R v Darrach, 2000 SCC 46 (CanLII),  2 SCR 443
R v Harris, 1994 CanLII 2986 (ONCA)
R v Herter, 2009 ONCJ 378 (CanLII) at para 1
R v Grey, 2013 BCCA 232 (CanLII) at para 42
- R v Darrach
R v DeClercq,  SCR 902, 1968 CanLII 24 (SCC)
Admissibility of Voir Dire Evidence Within the Trial
A voir dire is considered a separate hearing and so evidence admitted in the voir dire is not automatically evidence in the trial proper.
The evidence heard in a voir dire can become evidence of the trial proper where it has been found admissible in the voir dire and both parties consent (known as a "blended" voir dire).
Consent for a blended voir dire can be applied only to some of the evidence to the exclusion of the rest of the evidence heard.
However, where there has been evidence accepted at the voir dire that would otherwise be inadmissible at trial, and which was consented as applicable to the trial, the validity of the verdict may be in question.
The choice of whether to consent to a blended voir dire will have an effect on the manner in which evidence is presented. A non-blended voir dire may require the defence to lead evidence to establish the violation. A blended voir dire will require the evidence to be lead by the Crown.
R v Gauthier, 1975 CanLII 193 (SCC),  1 SCR 441 at 452
R v Viszlai, 2012 BCCA 442 (CanLII) at para 68
R v Erven, 1978 CanLII 19 (SCC),  1 SCR 926, at p. 932
R v Darrach, 2000 SCC 46 (CanLII),  2 SCR 443 at para 66
R v Dela Cruz, 2007 MBCA 55 (CanLII), 220 CCC (3d) 272, at para 24
R v Gauthier, 1975 CanLII 193 (SCC),  1 SCR 441, at p. 454
R v Sadikov, 2014 ONCA 72 (CanLII) at para 30
R v Jir, 2010 BCCA 497 (CanLII) at para 10
R v Ballendine, 2011 BCCA 221 (CanLII) at para 84
R v Dela Cruz, supra at para 26
e.g. R v Smith, 2016 BCSC 1725 (CanLII), at paras 45 to 47
R v Wilson, 2011 BCCA 252 (CanLII) at para 71
- See Charter Applications for details on burden