Kristen Bell will ''never forget'' witnessing the 9/11 tragedy as she worked on the stage play 'Reefer Madness' at the time.

The 38-year-old actress - who was in the midst of preparing for her Off-Broadway production - has opened up about seeing first-hand how the world was brought to a standstill on September 11th, 2001.

She recalled: ''We rushed outside and walked to the theatre in time to watch the second tower fall from the street. We tried to donate blood, then we went to our producer Stephanie's apartment and stayed there for the next 24 hours.

''We just waited to figure out what was happening. It was scary, and I'm still grateful that I had a safe group of people to be with. It's one of those things you never forget.''

The 'Forgetting Sarah Marshall' star explained how the experience created a special bond amongst the cast and crew.

She told Vulture: ''I just love those people so, so much. I have a very special relationship with that film.

''We were all together during the event, which bonds you to people, dare I say more, than being in a show with someone, when you're spending 12 hours together every day for six months at a time.''

Despite the devastation that surrounded the city, production of the movie-musical - which was a comedy remake of the 1936 classic cult film - opened four days later.

The actress shared: ''We still ended up opening the show. We agreed that we didn't know what people were going to want, but we had to keep our minds busy.

''We thought it might bring some people joy, and figure we ought to do the job we'd been paid to do. For about three weeks, we performed in a 500-seat theatre to an audience of four people.''