“All of the witnesses that are in this court room that are going to give evidence in this matter will go outside the court room and will remain there until such time as they are called. And they will not communicate with any witnesses who have given evidence and gone out of the court room.”
One limitation on the principle of an open court is the practice as to exclusion of witnesses.
No rule of law requires in a trial that the witnesses to be called by one side must all remain out of the court until their turn to give testimony arises. This is purely a matter within the discretion of the court.
Moore v. Lambeth County Court Registrar,  1 W.L.R. 141.
In criminal cases the trial judge may order any witness, other than the accused, excluded from the court room. Unless otherwise directed by the judge, such a witness must remain outside the court room until all of the evidence, including that given in rebuttal is completed.
If a witness nevertheless remains in court:
(a) he is not necessarily disqualified, although, in certain circumstances, the trial judge may exclude his evidence;
(b) the weight, if any, to be given to his evidence is for the jury, or for the judge, if there is no jury, to decide.
R. v. Dobberthien, 1974 CanLII 184 (SCC),  2 SCR 560.
A witness who remains in Court, disobeying an order for their withdrawal, may be cited in contempt.
Prejudice is presumed where a witness has heard a previous witness’ evidence in violation of an exclusion order. When this occurs, a caution to the jury is necessary.
R v Dobberthein, 1974 CanLII 184 (SCC),  2 SCR 560 .
The traditional justification for excluding witnesses is the danger that if the hearing of opposing witnesses were permitted, the listening witnesses could ascertain the points of difference between their testimonies and could shape their own testimony to better advantage.
However, beyond the danger of a witness intentionally tailoring her evidence in light of the evidence she has heard, there is the risk that her evidence may be influenced or compromised by the overheard evidence in other ways. As the Supreme Court of Canada noted in R. v. Dobberthien (Ibid.), “The influence of the overheard evidence may be multifaceted, nuanced, pervasive and difficult to identify and therefore difficult to effectively overcome.”
Written by Stuart O’Connell (Barrister/Solicitor).