Expectation bias, fueled by familiar precursors to a “line up and wait” clearance, led this B737 flight crew to enter the runway prematurely. We were holding short of Runway 06L and takeoffs and landings were being conducted on the runway. The Captain had mentioned that he had a commute to catch at [our destination] and we were issued a wheels up time. The aircraft ahead in the run-up area was cleared for takeoff. I glanced right, saw the next arrival for the runway and thought we might be able to get out before him if we got clearance right now. The Captain released the parking brake to inch forward to the Hold Short line since the aircraft ahead had departed. As we were rolling, the Tower Controller issued instructions to amend our departure. I read them back and then focused my attention on the automation to reset the departure….
As I looked back outside the aircraft, I saw that we were lining up on the runway. As my focus had been inside the airplane, I did not immediately perceive any error. I then tried to think back whether we had been cleared to line up. As we lined up, ATC instructed another aircraft to go-around. It then clicked that we had never been cleared to line up and wait. The Captain then also realized his error. Some factors included how ATC worded the departure amendment in a way that sounded like the precursor to a line up and wait or takeoff clearance. Another was glancing at the next arrival. Since our wheels up time had come, my mindset was that we were next and had enough room if we got clearance to takeoff right away. When ATC issued the departure amendment, the aircraft was already rolling forward as my head went down. I felt aircraft movement because we had been creeping forward, but did not realize how far we had gone before putting my head back up.Source: http://asrs.arc.nasa.gov/docs/cb/cb_418.pdf