Michelle Williams equates her holiday horseback experience to "being piggy-backed by a drunk person."

The My Week with Marilyn actress vacationed in Mexico over the Christmas holidays.

Michelle and friends decided to try horseback riding.

The actress has some regrets about the choice.

"I likened it to being piggy-backed by a drunk person, [someone] who has no personal investment in you and will throw you at any time to save themselves," Michelle joked on late night TV show Jay Leno Thursday. "I'm still a little sore."

Michelle was not aware that the ride would be so rigorous.

"I thought it was going to be one of those slow ambles through the hills, and you take a lot of photos and take in the beautiful scenery," she explained. "[Instead] we went down ravines; we went trekking. It was an eight-hour horseback ride."

During the equestrian expedition, Michelle and party took photographs on the edge of a cliff while mounted on their steeds.

The star was worried about the precarious position she was in, but her tour guide assured her that all was well.

"[My guide] said, 'I promise you there's no such thing as a suicidal horse.'" she recalled.

My Week with Marilyn was released in US theatres last month.