Producers approached Willis about starring in the action film during the 1980s, but the hunk was already too busy filming his hit Tv series, about an investigator and ex-model running a detective agency.
Yet in 1987, Shepherd discovered she was expecting twins, forcing creator Glenn Gordon Caron to give the cast several months off since her pregnancy didn't fit the storyline.
He tells U.S. Tv host David Letterman, "They called me a couple times when I was doing Moonlighting and I said, 'I can't do it fellas, I just can't do it,' I was doing my show and thank God Cybill Shepherd got pregnant. Glenn Caron gave us off 11 weeks and I went to go do Die Hard."
Willis is currently promoting the fifth film in the franchise, A Good Day to Die Hard, which hits U.S. theatres on Thursday (14Feb13). So far the series has raked in $423 million (£264 million) in global ticket sales.