After 6 playthroughs ... I think it's fair to say I've seen a good range of these transitions, but there have been some mentioned here that I haven't seen! Which I think is amazing.
I've only seen Trevor flushing a human foot down the toilet the once. Although, there was a period of time where all he seemed to do was jack off in his bathroom. I mean, about six consecutive times when I switched to him, there he was, finishing off and wiping it on the wall. I also got the scooter brothers one quite frequently on, I think, my second playthrough, and right now, he's going through a phase of rooting through and eating people's trash. But most of them I've had happen more than once - it's always fun to see what Trev's getting up to.
Michael, most often seems to be either coming out of Clappers after a good meal, or walking away from a motel claiming he 'couldn't go through with it' (or on one occasion that his 'dick might fall off'). Or gazing at the sunset at the end of Del Perro pier, enjoying a cigarette by the marina ... or, in Sandy Shores, on a boat, cruising around the Alamo Sea. Got to drop in on him giving advice to a crew member at the movie studio the other day, though, which I don't recall happening before. After the end of the game, he pretty much just sits by his pool reading screenplays. I quite like the ones where he's spending some quality time with his family, though; having lunch with Amanda or watching TV with his kids.
Franklin, for me, is always cleaning his bike up in Vinewood hills, or coming out of a smoke shop, mostly. Yeah, in the same boat here - for a while, he pretty much just ate crisps in his mansion. Like with Trevor, he almost constantly seemed to be doing this. Have had him walking Chop a few times, though, too, or just riding around on his Bagger. I've seen him arguing with Tanisha a couple of times, now, though; once, I think, via phone call (not after the Jewel Store Job), though I can't be sure. And yeah, seeing off a booty call in a taxi, too.
Still find it fun switching to different characters, purely to see what they're getting up to. Haha.