The 27-year-old star - who is currently starring in hit play After Miss Julie in New York - has had to deal with a string of bizarre distractions from the audience, something you don’t get when shooting a movie.
She said: The other night there seemed to be someone in the front row convulsing.
Then he lay down and I thought he had passed out. I stayed in character and carried on, which was just as well because it turned out it was just a guide dog simply shaking itself out before going to sleep.
Although Sienna is coping well so far, she admits it can sometimes be tough staying professional.
She added to Britain’s Daily Express newspaper: Another time, a woman in the crowd was making so much noise it was as if she was eating sweets out of a giant trash bag - I nearly told her to shut up.
But it’s as well I didn’t because she was ill, passed out soon afterwards and had to be taken to hospital.
Sienna also revealed another trick she has learned during her career always have a failsafe look for the red carpet.
The 'G.I. Joe: The Rise of Cobra' actress said: "You should always have a reliable big-event look. When I need to feel comfortable for the red carpet, I stick to my failsafe look of smoky eyes and nudge lips. I use black eyeliner, a volumising mascara and a nude gloss."
