Federal Rule of Evidence 615 provides that:

At a party’s request, the court must order witnesses excluded so that they cannot hear other witnesses’ testimony. Or the court may do so on its own. But this rule does not authorize excluding:

(a) a party who is a natural person;

(b) an officer or employee of a party that is not a natural person, after being designated as the party’s representative by its attorney;

(c) a person whose presence a party shows to be essential to presenting the party’s claim or defense; or

(d) a person authorized by statute to be present.

Many states have evidence rules modeled after or even identical to the Federal Rules of Evidence. The Evidence blog recently had this post:

“If Elaine and Felicia are eyewitnesses to a murder and witnesses against the defendant at trial, defense counsel can move to have Elaine excluded from the courtroom when Felicia testifies and vice versa. The concern here is that a later witness might hear the testimony of an earlier witness and, consciously or unconsciously, tailor her testimony to the testimony of the earlier witness (e.g., Felicia might plan on saying that the day of the murder was a sunny day before hearing Elaine testify that it was a cloudy day).

Judges often expand the scope of sequestration under Rule 615. For instance, in United States v. Smith, 2017 WL 3393934 (6th Cir. 2017), the judge issed a “courtroom procedures and decorum” order under Rule 615, which stated that:

[i]f witnesses are sequestered, counsel must assure that each witness called…understands that (s)he may not discuss the testimony (s)he expects to give or has given in the matter before the court…[and] should anyone attempt to discuss the testimony (s)he has given or expects to give…(s)he may not engage in such discussion.