Speaking to crowds at the West Hollywood's Harmony theatre during a Q&A for Cake, the 45-year-old actress revealed she had the "worst day of her life" after she was forced face a fear she's had since she was a child - going under water.
"As a kid I was with my brother and [I] was driving my tricycle around the swimming pool and drove my tricycle into the swimming pool and I didn't let go," Aniston said.
After the crew and cameramen failed to believe she was truly scared, Jen was left to brave the waters for her role as Claire Bennet, which required her to be submerged in a pool.
"Just to get the one shot of me going under [water] with the weight, I think we did that 30 times," Jen added.
Cake tells of a woman's struggle with chronic pain, a role that Jennifer said she was desperate to play and had been "dying to for years". "It was honestly the first time for me to be able to do a true study of a character," she revealed.
Already nominated for Best Actress in this years Golden Globe awards, she is also tipped for an Oscar nomination. Could this be the awards season where we finally see Jennifer Aniston outgrow Rachel Green?