The 33-year-old gained 20 kilograms (45 pounds) to play overweight MURIEL HESLOP in 1994 comedy Muriel's Wedding, and was amazed when she was able to put on 12 kilograms (27 pounds) a decade later.
But Collette is wary about putting her body under such immense strain again as she grows older and her metabolism slows down.
She says, "Curtis Hanson (director of In Her Shoes) asked me to put on 45 pounds, which is what I put on for Muriel's Wedding, and I said, 'Man, that was 10 years ago. I don't think I can do it now.'
"But I just went for it. I found out just before Christmas, which was the perfect time to indulge.
"It ended up being 27 pounds, which is still a lot.
"It takes a long time for your body to get back to normal, and it's not something I'm eager to do again."