"She had to sort of stamp around with Matt [Morrison] and the kids in a full tank of water for 12 hours," Murphy told PEOPLE at the 24th Annual Emery Awards in New York on Tuesday. "She loved it, actually. She's just the best."
Paltrow, who is set to perform at Wednesday's CMA Awards, will sing a mash-up of "Singin' in the Rain" and Rihanna's hit "Umbrella" on the episode airing Tuesday (8 p.m. ET).
It didn't take much work for Murphy to convince his "good friend" Paltrow to show up at McKinley High. "I've always loved her when she sang. And I said, 'You should sing and dance, and show people what you can do,' " Murphy says. "She has a movie coming out in December, Country Strong, so she said, 'Oh, that might work.' "
And the actress had some special visitors on set – daughter Apple, 6, and son Moses, 4. "Her kids came and watched it. She had a ball," Murphy says. "She's the easiest, breeziest gal around, she's such a professional. She was so prepared, she worked so hard."