Boyle's showstopping audition became a YouTube sensation last year and she has gone down a storm in the United States, gathering a string of celebrity fans.
Actress and TV presenter Holden, 39, told Good Housekeeping magazine how Boyle changed the nature of the contest.
She said: "Last year's series became iconic - you're not going to find another Susan Boyle in 10 years.
"We had no way of predicting that she would turn into a global phenomenon.
"But it has changed the nature of the show, as now, potentially, we are looking for a worldwide star.
"I just have to go on my gut instinct, thinking: 'What do I like? And what would my mum and nan like?'"
Holden also spoke of her hopes to have another child.
Talking about her four-year-old daughter Lexi, Holden said: "I'm just like any other working mother in the country - dealing with the same mix of guilt and worry."
She later added: "I don't really want Lexi to be an only child, but at my age I think I'm very lucky to have one healthy one."