updated 01/06/2012 AT 2:00 PM ET
•originally published 01/06/2012 AT 3:00 PM ET
When Michelle Williams took a holiday vacation to Mexico, she may have gotten a little more than she bargained for.
“I’m still a little sore,” she told Jay Leno on Thursday’s episode of The Tonight Show when asked about her trip.
The My Week with Marilyn star was vacationing with a group of friends when they decided to try horseback riding.
“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 said. Instead, “We went down ravines; we went trekking. It was an 8-hour horseback ride.”
“I likened it to being piggy-backed by a drunk person,” Williams continued, “who has no personal investment in you and will throw you at any time to save themselves.”
Williams, who once considered herself an experienced rider, felt especially frightened when it came time for a prime photo opp – at the edge of a cliff. Her tour guide assured her she had no reason to worry.
“He said, ‘I promise you there’s no such thing as a suicidal horse.’”
[RELATED_TEXT “Can’t get enough pet news? See more on PEOPLE Pets