Orders to Exclude Witnesses from Court
A judge has the discretion to order the exclusion any witness from being present in court when other evidence is called.
The power to exclude witnesses (sometimes called "sequestration of witnesses") is for the purpose of "detect[ing] falsehood by exposing inconsistencies", and prevents one witness from being "taught" by another witness. And it prevents the risk of a witness to "alter, modify or change" their evidence due to hearing another witness testify and is particularly important where credibility is in issue. 
It has also been said that the purpose is for the purpose of:
- preventing the prospective witnesses from consulting each other;
- preventing them from hearing a testifying witness; and
- preventing them from consulting a witness who has left the stand.
A violation of an exclusion of witnesses order will permit a judge to consider how that may affect the Weight of the evidence and may instruct the jury on the wait.
Where exclusion of witnesses order is violated, the judge cannot prohibit that witness from testifying, but may render the witness liable for contempt.
The exclusion of witnesses order will usually be read out in court by the judge or clerk in a form such as the following:
EXCLUSION OF WITNESSES
BY DIRECTION OF HIS LORDSHIP (or HIS HONOUR) ALL WITNESSES IN THIS CASE WITH THE EXCEPTION OF ..... (as directed by the presiding judge) ..... ARE TO BE EXCLUDED FROM THE COURTROOM UNTIL CALLED. IF YOU ARE BEING CALLED AS A WITNESS YOU WILL LEAVE THE COURTROOM AT THIS TIME AND TAKE YOUR PLACE IN THE WITNESS ROOM (or the usual place depending upon the accommodation available) AND BE AVAILABLE TO COME TO THE COURTROOM WHEN REQUIRED. WHILE WAITING TO BE CALLED YOU WILL NOT ATTEMPT TO COMMUNICATE IN ANY WAY WITH ANY WITNESS WHO HAS PREVIOUSLY TESTIFIED IN THIS CASE.
Effect of Breach of Exclusion Order
Where a witness hears evidence of a previous witness in violation of an exclusion order, it is presumed there is prejudice for the purpose of a mistrial application.
R v Leitner, 1998 CanLII 13871 (SK QB), at para 14
R v Hoyt (1949), 93 CCC 306 (N.B.S.C. App. Div.)(*no link)
R v Dobberthien, 1974 CanLII 184 (SCC),  2 SCR 560
- Regina v O'Callaghan, 1982 CanLII 2144 (ON SC)
- R v B.L.W.D., 2008 SKPC 56 (CanLII)
- Wigmore on Evidence, 3rd ed. (1940), vol. VI, at p. 361
R v Dobberthien,  2 SCR 560, 1974 CanLII 184 (SCC) - overturned CA who said judge had discretion
see also Chandler v Horne (1842), 2 M. & Rob. 423, 174 E.R. 338 (UK)(*no link)
Cobbett v Hudson (1852), 22 L.J.Q.B. 11 at p. 12 (UK)(*no link)
R v Carefoot, 1948 CanLII 34 (SCC),  O.W.N. 281,  2 D.L.R. 22, 90 CCC 331 (H.C.J.)
R v Briggs, (1930), 22 Cr. App.R. 68 (UK)(*no link)
R v Wilson (1973), 14 CCC (2d) 258 (N.S.S.C. App. Div.)(*no link)
- Regina v O'Callaghan, 1982 CanLII 2144 (ON SC) citing Wigmore on Evidence
R v Dobberthein, 1974 CanLII 184 (SCC),  2 SCR 560
R v Donszelmann, 2014 ABQB 255 (CanLII), at para 5