The expert witness is in the courtroom to provide their independent, expert opinion on the matter at hand. Their top priority is always to provide their truthful, impartial opinion—regardless of if their opinion benefits the party paying their fees. Additionally, the expert witness may be expected to provide a written report for the court attesting to their opinion and findings.
However, there are also a number of things that expert witnesses are NOT expected to do. Expert witnesses are not in court to advocate or argue for a particular side or a specific outcome for the case, nor are they there to give an opinion on anything outside of their narrow area of expertise. Additionally, expert witnesses are not there to advise what evidence or information should or should not be presented to the jury.