Ellen DeGeneres knew Simon Cowell could be harsh -- but she says she had no idea just how critical he could be until she recently taped an episode.

"He's actually meaner than I thought," she tells Entertainment Tonight.

"It's hard to listen to him tell people things and me not to go, 'You poor thing!'"

Reports surfaced last week that Cowell, 50, was already clashing with DeGeneres, 52, who makes her Idol  debut Feb. 9.

DeGeneres also is "getting good at delivering bad news" to Idol wannabes, she says.

"I'm trying to do it in a tender way," she tells ET. "I hate hurting people's feelings, so that, I think, is the hardest part."

She says that there are "good people" this season, and she's had to "fight very, very hard to keep the people that I believe in."