It is commonly understood that despite the fact that one party retained and paid for the services of an expert witnesses, experts are supposed to testify impartially in the sphere of their expertise. See Selvidge v. U.S. 160 F.R.D. 153. However, the court in Wilson v. City of Chicago stated that the Federal Rules of Evidence do not require experts to be impartial. (6 F.3d 1233, 1238).
If experts are not advocates, at the very least they advance the cause of the party that retained them – otherwise they would not be there. By the time of the trial, the retaining attorney knows how the expert will testify. It is unlikely that the retaining attorney would call the expert if the expert did not testify in favor of the retaining party.