It is a truism to say that evidence is what establishes a case. Most EHPs could list the sorts of evidence that can be presented in court without difficulty - oral testimony, real evidence, documentary evidence and so on. In some cases however, the most valuable evidence is not that carefully prepared before the case commences, rather it is evidence that emerges during examination-in-chief or cross-examination of the witness in the case, being evidence of reaction or behaviour.

In his instructions to the jury, the judge will often tell them not to take notes, but to watch the person giving evidence, and note their behaviour and reactions. Someone who goes deathly pale and blusters when confronted with a piece of previously unseen evidence may have something to hide, equally someone who is very smooth and unruffled may not. It is for the jury then to decide whether a witness is genuine but terrified of the whole process or an accomplished liar who has prepared his performance well.

It is not just the behaviour of the witnesses that may influence a case. Newspapers regularly carry stories about the reaction of the family and friends of the victims in court cases, and their reaction to the appearance of the defendant, the verdict or the adequacy of the sentence handed down. This behaviour generally has little influence on a jury, since it can hardly be considered to be a neutral view.

The behaviour of the other parties in the court, more intimately connected with the case can influence the jury or magistrates. Lawyers have their behaviour down to a fine art - the slow raising of an eyebrow or the removal of a pair of glasses is not just part of a fine piece of theatre, but is intended to suggest to the court that the lawyer does not believe a word of what he has just heard, and neither therefore should the court. For that reason, lawyers have to be measured, and, notwithstanding what appears to be common understanding of their role in television programmes, they should not influence the court with gestures or comments. What about the behaviour of other parties? What are the jury to make of policemen who fall about, convulsed with laughter, as the defendant puts his case? They are bound to believe that the police know it to be fiction and possibly take the same view. An EHP who has given evidence and sits in court may find the evidence of the defendant incredulous, or the comments of the defence solicitor in mitigation farcical, and may feel frustrated in the extreme that he or she cannot counter it.

The temptation to go into eye rolling or head shaking mode is considerable, as it may be seen as the only way in which the evidence being advanced can be challenged. The temptation must be resisted because behaviour in court that may influence the magistrates or jury can form the ground for appeal. There is no legitimate mechanism whereby a defence solicitor can recall a prosecution witness to cross-examine them as to why they are shaking their head - and no defence solicitor in his right mind would seek to do so, given the answers that such a line of questioning may throw up. The better course of action is to appeal on the grounds that behaviour of the performing party may have influenced the magistrates or jury and to seek to have the verdict set aside and a retrial ordered.

Magistrates are generally more robust than juries. In a case of which I am aware, in such circumstances they indicated that, although they had been aware of the gesturing, their decision had not been influenced by it, and dismissed the appeal. It is more difficult to determine the effect such behaviour may have on a jury, and, given that the defendant is always entitled to the benefit of any doubt that exists, in such circumstances a retrial may be ordered. There are then two problems - the first is that the defence is thoroughly pre-warned of the totality of the prosecution case. The second is the very real prospect of a wasted costs order being made against the party whose behaviour brought the first trial to a close.

Prosecutors are not totally impotent - the trick is to get your retaliation in first. Proper briefing of the prosecuting lawyer such that avenues that may have been open to the defence are closed before they are explored is the best way forward. The defence must be made aware of the previous history of the defendant to ensure that they are aware of the danger of bringing it into play. Next month, I will consider how this can be done.

Julie Barratt is director of CIEH Wales