Alexandra Daddario worked to gain muscle mass for Baywatch

Actress Alexandra Daddario had to overhaul her workout regime in preparation for the Baywatch movie so she could bulk up.

The San Andreas star is a gym regular and sticks to a healthy diet, but she wanted her body to be more toned and muscular for her scenes in the famous red swimsuit, alongside leading men Dwayne Johnson and Zac Efron.

"Really, I've never done anything where I've thought about my body as much," the 31-year-old tells Women's Health magazine. "You are literally in a bikini the entire day. I even wore a swimsuit when I auditioned with Zac!"

Alexandra turned to Zac's personal trainer, Patrick Murphy, to help her shape up for the role with hour-long, high-intensity work outs four to five days a week.

"My normal workout routine, pre-Baywatch, was a lot of hot yoga," she explains. "I feel like it helps with anxiety, it helps with sleep, it was sort of more of a mental thing.

"For Baywatch, I started doing weight training, which I had never done before, and I saw a huge difference. It was kind of amazing, the transformation I went through."

She kept Murphy on as her trainer after wrapping Baywatch, and admits he has helped boost her body confidence.

"As I've gotten older, something that made a huge difference for me was being healthy," she says. "It's not about how much you eat... it's eating healthy food and eating when you're hungry, and not thinking about it so much."

And part of her lifestyle philosophy is to allow herself a sweet treat every now and then - an indulgence she refrained from while preparing for her latest movie role.

"I do have an ice cream habit that I sort of cut out for Baywatch and now I'm back on that!" she recently shared on Entertainment Tonight.