Verb
walk in on (third-person singular simple present walks in on, present participle walking in on, simple past and past participle walked in on)
(idiomatic) To enter suddenly or unexpectedly while something is happening; to intrude or interrupt by entering.
He accidentally walked in on me while I was undressing.
I came ready to fight Genghis Khan and I walk in on a shut-in playing the biggest Dungeons and Dragons game in history. Richard Kadrey
I've been on projects before where there's no rehearsal, and you walk in on set and that's literally the first time you've ever played the character, and then I've had times where there's been three weeks of rehearsal. I like both. Josh Hutcherson