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It thus follows that if the defence are relying on a particular defence, the accused or defence witnesses will usually have to give evidence of that defence. Just as a defence witness may sometimes assist the prosecution case, sometimes a prosecution witness may inadvertently assist the defence. For example, a witness may state that he saw the victim punch the accused who then punched him back – suggesting self-defence. Defence witnesses may also give such evidence, but few juries will accept such a defence if the accused does not testify in person and is not subjected to cross-examination.

Where it serves merely to corroborate other evidence, it is not generally problematic. However, there is greater cause for concern where it is used as the sole or primary basis for the prosecution’s case. Eighteenth and nineteenth-century lawyers were well aware of the fallibility of identification evidence, but the fact that such evidence was a frequent source of false convictions failed to lead to any formal safeguards. However, many judges, perhaps personally aware of the risk of false conviction, took to warning the jury of the possible dangers of convicting on identification evidence alone, or even went so far as to direct an acquittal where they thought the identification was too poor to sustain a conviction.

A typical direction will begin with an explanation as to which side bears the burden of proof, against what standard those elements will need to be proved. He will normally take the jury through the prosecution evidence and, importantly, point out any defence which that evidence discloses, even if the defence have not relied on that particular defence. For example, on a charge of murder, the defendant may plead self-defence but the evidence may also point towards provocation. In these circumstances, the jury will be directed to consider self-defence first, but, if that fails, they should proceed to consider provocation in the alternative.