Heather Morris did intense weight training before acting opposite Britney Spears in Glee.

The 24-year-old actress - who plays Brittany S. Pierce - starred alongside the Baby One More Time hitmaker in an episode of the musical comedy, where she covered I'm a Slave 4 U, but she had to hit the gym hard beforehand.

She said: "I wanted to be fierce. I was like a monster, training my ass off."

Despite her extreme fitness schedule before the Britney episode, the blonde beauty - who stars alongside Lea Michele (Rachel Berry), Dianna Agron (Quinn Fabray) and Matthew Morrison (Will Schuester) in the Fox programme - has insisted people can still be in shape no matter what they eat.

She added to Women's Health magazine: "Do whatever you like to do, eat how you want to eat, but just make sure you stay [active]. I take a lot of exercise cardio classes. I do a ton of squats."

Heather - who progressed from a minor role in Glee to a bigger part after impressing the writers - also revealed she is very similar to her character in the show.

She added: "I'm exactly like Brittany. I love to dance, I like to have fun. I want to make everybody laugh. I'd rather have innocence than be serious all the time."

- BANG! Showbiz