For example, on the film "White Men Can't Jump" starring Woody Harrelson and Wesley Snipes, the titular lead single by group "Riff" featured Woody Harrelson, Wesley Snipes & Rosie Perez, who plays Harrelson's girlfriend in the film, clowning around on the basketball court. In the song "Shy Guy" by Diana King which was featured on the Bad Boys soundtrack, the video had Will Smith and Martin Lawrence goofing around and dancing in the video as well.
One of my favorite instances of this was for the film "Magnolia" by Paul Thomas Anderson (The Master, Boogie Nights), where he actually filmed the video for Aimee Mann's "Save Me". After cutting on a scene, they would basically reshoot the scenes as part of the music video, with Aimee sitting by the cast members in that scene, singing the song. It was a brilliant idea, and was a fantastic video.
MORE AFTER THE BREAK
This was a genius movie to bring the actors in, I thought when I saw them back then, because A. it provides a little more entertainment value for the video, and it also increases the interest that someone might have to go see the movie. I never understood why more movies didn't do this. It can only help the chances of the film's box office. Now I realize that there are instances where actors may demand to be paid more. That's understandable and I don't think is THAT unreasonable, although it could be argued that by participating in the marketing of it as it pertains to the music video (and that is considered marketing, I would think), that it would only help your movie do better.
There are instances where actors have demanded large sums of money to do DVD and bluray commentary tracks, which is why often you will rarely see big stars do the commentary tracks. In some cases it's a scheduling thing, and they may be filming another movie or something, but my opinion is that if they really wanted to do this and money wasn't the problem, they'd find a way to do it unless it's a case of them not caring for the movie or the experience they had.
If I were these studios, I would figure a way to write into the contracts as part of their agreement, and you can tack on an extra million dollars or something if it comes to that, that the main actors agree to be in the lead music video that is released. That's the 1st single that is out for a movie, which often would be released in advance of the film coming out. That would serve as just more promotion, and it could be factored in to the aspect of promoting the film which actors are obligated to do.
This isn't as big a deal as it was in the 90's when music videos were much more popular. Back then you had MTV, The Box, MuchMusic, BET, etc that were playing music videos, so often studios would put more money into the video in hopes of getting some extra ticket sales. Nowadays? Not so much. In fact I don't recall the last big movie soundtrack, although I'm not as up on the music scene as I used to be.
Below I've included some music videos for movies that featured members of the cast of said movie. Reminisce back to when this was a bit more common place, and just imagine if studios were smart enough to start this again.