But it's easy to cross the line into coaching/woodshedding a witness. An article on the Oklahoma Bar Journal website says it well:
While appropriate to encourage a witness to look at the jury or judge, to speak clearly and act confident while testifying, going much beyond that may lead to unintended consequences. A suggestion that a witness who may not be all that certain of a particular fact to change that demeanor in order to appear more assertive may very well cross that boundary from simply “preparing” a prosecution witness to impermissible “coaching” or “wood shedding” of that witness.
The Dallas Morning News has an article today on the case against Richard Miles for murder. At the trial, a witness, Marcus Thurman, identified Miles as the man who shot the victim. In a sworn statement taken this January, Thurman now says he didn't identify Miles because he recognized him as the shooter. In fact he says he didn't recognize Miles. Thurman says he identified Miles in court because the prosecutor, Thomas D'Amore, coached him to do so.
I've seen Thomas D'Amore around the courthouse but I don't know him, so I have no way of shedding any light on whether this is likely to be true or not. I can say that if this is true it's beyond unacceptable.