Surprisingly, Sandra Bullock wasn't director Alfonso Cuarón's first choice to star in "Gravity." (Twitpic)

But directors often have to go through a long process of casting and recasting with actors backing out for a variety of reasons (scheduling, creative differences, etc.) before all of the roles are filled and filming can even begin.

Here are 7 stars that were almost cast in some of this year's biggest movies!

Robert Downey Jr.

"Iron Man" star Robert Downey Jr. was director Alfonso Cuarón's first choice to play astronaut Matt Kowalski in his sci-fi thriller "Gravity," but he ultimately passed on the film and George Clooney was chosen to replace him.

Another one of this year's big movies that Downey Jr. was almost cast in was Sam Raimi's "Oz: The Great and Powerful."

Blake Lively was passed on for the lead female roles in "Gravity" and "The Great Gatsby." (Twitpic)

"Gossip Girl" actress Blake Lively was in talks for the role of Daisy Buchanan in Baz Luhrmann's "The Great Gatsby," but was ultimately passed over for Carey Mulligan, who had amazing chemistry with Leonardo DiCaprio.

Although other equally talented A-listers, such as Keira Knightley and Natalie Portman, auditioned, we don't think we could imagine anyone else playing the tragically beautiful and whimsical Daisy of Jay Gatsby's dreams.

Lively was also one of the many actresses to audition for the lead female role in "Gravity," which ultimately went to Sandra Bullock. 

Hugh Jackman

Finding an actor to play John F. Kennedy in "The Butler" proved to be a difficult task for director Lee Daniels. Although Hugh Jackman was originally cast, plans fell through and he had to be replaced.

Daniels then cast Matthew McConaughey, who left to film "Dallas Buyers Club," before finally getting Jackman's fellow "X-Men" co-star James Marsden to step in as the 35th president.

But don't be too bummed out for Daniels though - he gets another shot at working with Jackman on his next project "Orders to Kill," a film about the assassination of Dr. Martin Luther King Jr.

Mila Kunis

The role of JFK wasn't the only one Daniels had to find multiple replacements for, his wife Jackie Kennedy was also difficult to cast.

The director's first choice was "That 70s Show" actress Mila Kunis.

However, like Jackman, she had to be replaced, but this time Daniels got lucky on his second try, casting Minka Kelly as the First Lady.

Benicio del Toro was too expensive for J.J. Abrams. (Twitpic)

Benicio Del Toro

Director J.J. Abrams' first choice for the villain in "Star Trek: Into Darkness" was Puerto Rican actor and film producer Benicio del Toro.

Unfortunately, del Toro was too expensive so Abrams had to turn to other actors in order to replace him.

Since the character of Khan is notable for his Latino accent, other Hispanic actors were considered, including Jordi Molla and "Wrath of the Titans" star Edgar Ramirez. In the end, however, English actor Benedict Cumberbatch was cast.

Emma Roberts was cast in "Spring Breakers" before Ashley Benson replaced her. (Twitpic)

Emma Roberts

Originally cast to play Brit in Harmony Korine's raunchy comedy "Spring Breakers," Emma Roberts backed out due to some "creative differences."

We're thinking it might have something to do with the many crazy things her character was supposed to do, including a three-way sex scene.

The "American Horror Story: Coven" star was then replaced by Ashley Benson.

Angelina Jolie

Originally, Angelina Jolie was slated to play opposite Robert Downey Jr. in "Gravity." However, the deal didn't work out and Cuarón had to court multiple A-listers before ultimately casting Sandra Bullock as Dr. Ryan Stone.

Other actresses considered for the role include Scarlett Johansson, Marion Cotillard, and Natalie Portman.

Watch the trailer for "Gravity" below:

Reach Staff Reporter Michelle Tiu here or follow her on Twitter.