Katie stopped by the show to promote her Broadway play "Dead Accounts", in which she plays Lorna, the sister of a con man who brings home a ton of money, and she revealed that she actually recently lived out one of her worst nightmares: forgetting her lines.
"I have this kind of big speech in the second act," she says; "and it's pretty important because it kind of goes along with the theme of the show.
"I said the first line, and I'm looking at [co-star] Judy Greer and I'm supposed to be angry at her, and then the second line I didn't remember. And I didn't remember the third one."
Katie then showed the funny blank face she made (see it below) during the gaffe; saying, "I just looked at her like, awww!"
"So I just... obviously, I know what the point of it is. So, I kind of... made my way there," she explained.
Katie said the instance "felt like a good hour".
The newly single 34-year-old didn't discuss her recent divorce from actor Tom Cruise during the segment.