When Robert Downey Jr. had to step out of the starring role in Jon Favreau’s Cowboys and Aliens, I became slightly concerned that the movie might not work or sell as well without him as the bankable lead. Over the past few months my concerns have vanished as Favreau and co. have added star after star to fill the void.

First we found out Daniel Craig would be taking over the lead role of Zeke Jackson, followed by the notable addition of Olivia Wilde. Then the movie got a heck of a lot more star power when Favreau confirmed through Twitter that Harrison Ford joined Cowboys and Aliens as well.

While Favreau couldn’t get his Iron Man 2 star, he did just get an Iron Man 2 co-star in the form of Sam Rockwell who Heat Vision is now exclusively reporting has also joined the cast. Rockwell will play Doc, a bar owner who joins Zeke Jackson on the hunt for aliens.

The character was originally a smaller role designed for a heavy set man but when writers Alex Kurtzman and Roberto Orci discovered Rockwell was interested, they re-wrote the role a bit to give him more screentime.

I’m very excited at the idea of these actors playing cowboys, working together to fight… aliens. With Favreau at the helm, the cast involved and the writing team behind Star Trek working the screenplay, I think we have a very entertaining movie awaiting us next summer.

Cowboys and Aliens is a DreamWorks and Universal production set in the 1800s where Cowboys team up with Native Americans to combat a common unearthly foe. The story is an adaptation of the Cowboys and Aliens graphic novel series by Scott Mitchell Rosenberg.

What do you think about the latest cowboy entering the fray?

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Cowboys & Aliens begins shooting this summer and opens in theaters on July 29, 2011.

Sources: Heat Vision