I agree witnesses would have to be able to see the defendant and vice versa. But most witnesses who have received preparation for testifying look towards the jury or at least at the person asking the questions when testifying, not at the defendant. So I doubt witnesses, at least adult witnesses, will be rattled by her mouthing words or shaking her head during their testimony. LS
may be more extreme than some but an awful lot of defendants do things like that during testimony from state witnesses. A difference here is that jurors are able to ask questions and a juror asked about LS's behavior. (Not all states allow jurors to ask questions but it is apparently required in Colorado
Can Jurors Ask Questions During Trials?)
JMO