I'm sure it depends on the band. In bands I've been in, we all tended to work out our own parts; I wouldn't tell the drummer, guitarist, or anyone else to do something differently unless it was clearly wrong. But then, that's learning cover songs. It's pretty obvious when what you're playing isn't what's on the record.
When you're working up your own material, there's probably more give and take, and people can suggest different ways of playing/singing something. The only "right" way is what sounds best by consensus or vote, so it really is a matter of others merely suggesting things that might sound better. I hadn't really thought of that before, but it still seems like there's more footage of Mike telling others what to do than anyone else. Part of that is probably because he and JP were the producers and were doing that anyway, and the rest is probably because Mike also edited the footage, therefore we saw more of him than anything else.
I don't want to turn this into Mike-bashing, in fact I was kinda trying to steer it the other way. I've never been in a band that did much original material, and sure as hell have never been in a recording studio. Something tells me I'd be in for a hell of a shock the first time.