The director revealed that he originally had no idea how the iconic film would finish since he was uncertain of what would be found in the ark.
"I was a little bit dubious about what happens when they open the ark," Spielberg told Entertainment Weekly. "What actually is going to come out of the ark?
"There were a lot of crazy things in the script. I wasn't sure how much we could actually get on the screen. We made a lot of it up as we were in postproduction."
Spielberg is currently at work on Lincoln, which will see Daniel Day-Lewis take on the role of the 16th President of the USA.
The director's next film War Horse will open in the US on December 25 and January 13, 2012 in the UK.
Watch the trailer for War Horse below: