Sexy JESSICA SIMPSON is wearing full army fatigues and packing an M16 in her new comedy, 'Major Movie Star,' and ET's on the set!

In the movie, out next year, Jessica plays a famous actress who enlists to win the approval of a producer and studio who consider her unsuitable for a military-themed project -- and quickly regrets it.

"At the beginning of 'Major Movie Star' it is glamorous because I'm doing the whole red carpet and everything -- but you will definitely see a transformation," Jessica tells our own KEVIN FRAZIER. "I mean, pretty much everyday we've had to wear dirt and mud on our faces."

Remind you of another blonde at basic? Try GOLDIE HAWN in 1980's 'Private Benjamin.'

"I adore Goldie," says Jessica. "I think she has a career I would aspire to have. She definitely makes us blondes look really great."

Just like basic training, shooting the film has been no picnic, with temperatures reaching 104 degrees in the shade.

"Louisiana in the summer is hot," she explains. "I have two pairs of socks on right now, because I want to wear everything that the army would wear. So we're in full fatigues every single day in this heat. We've all lost a lot of water weight, that's for sure.

"I am here because I love what I do, but I'm also here to entertain," continues Jessica, who is also involved with the USO and has visited our troops in the Middle East. "You think it's hot here? Try being in Iraq. I have the utmost respect for men and women in uniform, and I'm here to show what basic training's all about; it's just me kind of paying my respect as much as I can."

On top of the heat, drill sergeant VIVICA A. FOX makes Jessica's life a living hell in the movie. But off-camera, Jessica says that reports that the two actresses are feuding are just untrue.

"People saying I don't speak to Vivica is really funny because Viv and I talk all the time," explains Jess. "We were just looking on her Blackberry at LOUIS VUITTON bags. Then we have to go hate each other when we're on set because, you know, that's us acting. A lot of people can't separate the two, I guess."

Watch ET for more with Jessica on the 'Major Movie Star' set!