Air of Reality Test
Before the trier-of-fact can consider a justification or excuse defence there must be an "air of reality" to the defence. Should there be an air of reality to the advanced defence, the burden is then upon the Crown to disprove at least one of the elements of the defence beyond a reasonable doubt.
The purpose of the air of reality test is to prevent "outlandish defences" being put to the jury that would be "confusing and would invite unreasonable verdicts."
As part of the trial judge's gatekeeper function, the judge must ensure that the trier-of-fact "does not become sidetracked from the real issues in a case by considering defences that the evidence cannot reasonably support".
Nature of Burden Upon Accused
The air of reality test creates an evidential burden, not a persuasive burden.
In the context of a jury trial, the test determines whether the judge will give instructions to the jury that they should consider the particular defences. The jury should be instructed only of the defences that have evidence supporting it. By inference, a "judge has a positive duty to keep from the jury defences lacking an evidential foundation". There must be evidence support each element of the defence.
The test requires that there must be "some evidence" upon which "a properly instructed jury acting reasonably could base an acquittal".
Evaluation of Evidence
The trial judge must assume that the defence evidence is all true. The judge should not consider credibility, make findings of fact, draw inferences, or "weigh" the evidence. He should not consider the likelihood of success of the defence.
The judge may perform a limited, common sense weighing of the evidence.
The judge must consider whether inferences would be necessary for the defence to succeed and whether those inferences from the evidence are reasonable.
Where the stories of witnesses differ, the trier-of-fact may "cobble together some of the complainant’s evidence and some of the accused’s evidence" to determine if there is a an air of reality.
There is no rule against putting an alternative defence theory to the jury that is factually incompatible with the defence's principal theory. The only applicable test is whether there is an air of reality based on the evidence.
Whether there is an air of reality to a defence is a question of law and is reviewable on a standard of correctness. 
- R v Cinous, 2002 SCC 29 (CanLII), (2002), 162 CCC (3d) 129 (S.C.C.), per McLachlin CJ and Bastarache J, at paras 53-54 and 65
e.g. comments of Watt J. In R v Tomlinson, 2014 ONCA 158 (CanLII), at para 51
- Cinous, supra
- Cinous, supra at para 84
R v Singh, 2016 ONSC 3739 (CanLII) at para 36 per Fairburn J.
- Cinous, supra at para 52 ("It is trite law that the air of reality test imposes a burden on the accused that is merely evidential, rather than persuasive.")
R v Ribic at para 38 (all defences "that are realistically available on the evidence")
Cinous, supra at para 50 (“a defence should be put to a jury if and only if there is an evidential foundation for it”)
- R v Gunning, 2005 SCC 27 (CanLII),  1 SCR 627, at para 29
- R v Ribic, 2008 ONCA 790 (CanLII), per Cronk JA, at para 38 ("if evidential support for a necessary element of a defence is lacking, the air of reality test will not be met.")
Cinous, supra at para 57 and 82
- Ribic, supra at para 38
Cinous, supra at para 83
Cinous, supra at paras 53
- Cinous, supra at para 54
- Cinous, supra at para 54 ("whether the defence is likely, unlikely, somewhat likely, or very likely to succeed at the end of the day")
R v Larose, 2013 BCCA 12 (CanLII) at paras 27 to 28
Cinous at para 65 and 83
R v Savoury, 2005 CanLII 25884 (ON CA) at para 45
R v Basit, 2013 BCSC 70 (CanLII) at para 7
R v Park, 1995 CanLII 104 (SCC),  2 SCR 836
R v Esau, 1997 CanLII 312 (SCC),  2 SCR 777
- R v Gauthier, 2013 SCC 32 (CanLII) at para 29
- Cinous at para 55
R v McRae 2005 CanLII 26592 (ON CA) at para 38 ("[T]he question of whether there was an air of reality to the defence of duress is an issue of law")
R v Ryan, 2011 NSCA 30 (CanLII) at para 114