At the end of 2008's Iron Man, fans were treated to the very first one of Marvel Studios' now-famous post-credits sequences. The scene saw Samuel L. Jackson, famously the visual model for the Nick Fury in the pages of Mark Millar and Bryan Hitch's The Ultimates (a major influence for Marvel Studios), let Robert Downey Jr.'s Tony Stark know that Iron Man wasn't the only game in town, noting, "You think you're the only superhero in the world? Mr. Stark, you've become part of a bigger universe. You just don't know it yet," and when Tony expressed confusion, Fury told him, "I'm here to talk to you about the Avenger Initiative." However, that was nearly the full extent of Jackson's tenure as Nick Fury.
The new book, The Story of Marvel Studios: The Making of the Marvel Cinematic Universe, reveals that Marvel Studios was playing it purely by ear with Jackson's cameo in that first Iron Man film, explaining, "[Iron Man associate producer Jeremy Latcham] says they called Jackson and asked if he was still interested, and if so, would he do a cameo for them. 'We didn't have a deal with him for future movies,' Latcham clarifies. 'It was just this weird idea that maybe people give a s—t if we stick it on the end.'"
The anecdote in the book continued to note, "Jackson agreed, and even if the scene didn't amount to much at the time, the team at Marvel Studios decided it should remain as secret as possible to try and preserve a surprise for the hard-core comic fans who would get it right away."
Since Iron Man, Jackson's Nick Fury has appeared in a number of other otherwise standalone superhero films (Thor and Captain America: The First Avenger) before the Avengers Initiative kicked into high gear with 2012's The Avengers. The MCU has become a major force in our popular culture over the last decade-plus. After the success of Iron Man, Marvel Studios quickly worked out a deal with Jackson to do another nine films, but there was a real possibility that that cameo was all that we would have ever seen from Jackson.
Source: The Story of Marvel Studios: The Making of the Marvel Cinematic Universe, via ComicBook.com