The actor was appearing in the Arthur Miller classic with Scarlett Johansson on Friday night (26Feb10) and was waiting in the wings when he heard a woman calling out for assistance from the crowd.
Schreiber reportedly rushed down into the audience to help the unnamed female, who was slumped in her seat, according to TMZ.com.
A witness tells the website the actor "sprung to action" and a doctor in the audience was able to treat the woman until paramedics arrived.