The 33-year-old decided to slim down when he discovered he would have to appear shirtless in the movie, reports contactmusic.com.
"It is R-rated and I'm not in shape for no reason. This time when I take my shirt off, I've made a promise to myself, it doesn't have to be funny," the "How I Met Your Mother" star told Us Weekly magazine.
Segel features opposite actress Cameron Diaz in "Sex Tape".
Photos: Shraddha Kapoor, Kim Sharma at 'Haseena Parkar' screening
Mumbai to Goa train: First look at the glass-top Vistadome coach
Shuttler Ashwini Ponnappa keeps it short and sexy on social media
Photos: Narendra Modi, others at Marshal Arjan Singh's funeral
Photos: Sussanne Khan and Nimrat Kaur spotted at a spa in Juhu