Ideas for a film based on the Avengers began in 2003, with Avi Arad, the CEO of Marvel Studios, first announcing plans to develop the film in April 2005, after Marvel Enterprises declared independence by allying with Merrill Lynch to produce a slate of films that would be distributed by Paramount Pictures. Marvel discussed their plans in a brief presentation to Wall Street analysts; the studio's intention was to release individual films for the main characters—to establish their identities and familiarize audiences with them—before merging the characters together in a crossover film. Screenwriter Zak Penn, who wrote The Incredible Hulk (2008), became attached to the film in 2006, and was hired by Marvel Studios to write the film in June 2007. In the wake of the 2007–2008 Writers Guild of America strike, Marvel negotiated with the Writers Guild of America to ensure that it could create films based on its comic book counterparts, including Captain America, Ant-Man and The Avengers. After the successful release of Iron Man (2008) in May, the company set a July 2011 release date for The Avengers. In September 2008, Marvel Studios reached an agreement with Paramount—an extension of a previous partnership—which gave the company distribution rights for five future Marvel films.Casting began in October 2008 with Downey's signing. Though Don Cheadle was also reported to be reprising his Iron Man 2 role of War Machine for The Avengers, he later stated that he did not think the character would appear in the film. At the same time, two major prospects occurred for Marvel: Jon Favreau was brought in as an executive producer for the film, and the company signed a long-term lease with Raleigh Studios to produce three other big-budget films—Iron Man 2, Thor (2011), and Captain America: The First Avenger (2011)—at their Manhattan Beach, California complex. Lou Ferrigno, who voiced Hulk in The Incredible Hulk, stated that he would be involved in the film. In February 2009, Samuel L. Jackson signed a nine-picture deal with Marvel Entertainment to play Nick Fury in Iron Man 2 and other films. In September 2009, Edward Norton, who played Bruce Banner in The Incredible Hulk stated that he was open to returning in the film. The next month, executive producer Jon Favreau stated that he would not direct the film, but would "definitely have input and a say". Favreau also expressed concerns, stating, "It's going to be hard, because I was so involved in creating the world of Iron Man, and Iron Man is very much a tech-based hero, and then with Avengers you're going to be introducing some supernatural aspects because of Thor... [Mixing] the two of those works very well in the comic books, but it's going to take a lot of thoughtfulness to make that all work and not blow the reality that we've created". In March 2009, actress Scarlett Johansson replaced Emily Blunt in portraying Natasha Romanoff in Iron Man 2, a deal that subsequently attached her to The Avengers. The following day, Marvel announced that the film's release date had been pushed back to May 4, 2012, almost a full year later. Chris Hemsworth and Tom Hiddleston joined the film's cast in June, returning as Thor and Loki, respectively.In July 2009, Penn talked about the crossover process, stating, "My job is to kind of shuttle between the different movies and make sure that finally we're mimicking that comic book structure where all of these movies are connected. . . There's just a board that tracks 'Here's where everything that happens in this movie overlaps with that movie'. . . I'm pushing them to do as many animatics as possible to animate the movie, to draw boards so that we're all working off the same visual ideas. But the exigencies of production take first priority". At first, Penn tried to reduce Thor's role in the script because he had doubts about the character's ability to succeed on film. He changed his mind once Hemsworth was cast as Thor. The film had always intended to use Loki as 

Question: What year was Iron Man released?
Answer: 2008

Question: Who did Scarlett Johansson replace?
Answer: Emily Blunt

Question: When did casting begin?
Answer: September 2008