was caught with his pants down — by !

The  nominee appeared on Thursday, where he revealed he got a surprise visit from Kidman at a recent showing of his London production of , in which he plays Stanley Kowalski.

Mescal, 27, told  he didn't know Kidman, 55, was at the show. He was sharing a dressing room with two castmates, and they heard a knock on the door while they were changing.

"We thought it was the stage manager or something and I go to open the door," the  actor recounted. "It's , and I'm ."

Mescal continued sheepishly, "You'd think the next smart step is [to] put on some trousers, but there's something incredibly undignified about the act of [pulling up pants]."

"It's not a look you want to be caught in, [bending] over," he added.

After he shared a half-dressed hug with the Oscar winner and she left, Mescal said of himself and his fellow dressing-room inhabitants, "You just turn to each other like, 'What the hell just happened?' "

"When  is in her seat at a play, is she talking to the screen about the magic of film?" asked a straight-faced Kimmel — a nod to  — as his guest and the audience laughed.

Kimmel, 55, is set to host the upcoming 95th Academy Awards, at which Mescal is  for his role in .

While speaking with PEOPLE recently ahead of the upcoming ceremony, Mescal said there are "going to be just so many  on March 12.

"I'm, like, a crazy  fan, so that's gonna be — I feel like I would find it difficult to even talk to Michelle Williams, so that's a big one," said the  star. "[I'm looking forward] to maybe getting to know  a bit better, bumping into him over the next couple of weeks."

"There's probably a long list of people I'm looking forward to meeting," added Mescal, who said  for the show.

"Bumped into  the other day," he also told PEOPLE about his recent celebrity interactions during awards season. "Pretty wild — she said that she loved the film. I didn't really get much words out of my mouth. I think there will be a lot of that, kind of pinching myself."

"I think it's really cool that people I really admire are seeing the film," Mescal added of . "I actually didn't know that it was the actor's body of the Academy that vote for the actors, and I think that's something I take great pride in, that actors are enjoying films like  and my work."