I find during play to be generally more fluid but YMMV.
The reason I don't like to do this during play is that I find it causes people to drop out of character too much and continually think in the meta-game. I really like players to be thinking "Alright, I'm on a street, I'm a badass rogue who owes money to everyone in the city. I see people in the shadows...what would I do?" rather than "Oh, wouldn't it be awesome if the people in the shadows were members of the Shadow Cartel and they want me dead because of a job I failed to pull off for them 2 years ago? Hey, DM. Can the people in the shadows be that group?"
I've likely already figured out who the people in the shadows are and why they are there and it is way too late to start giving the PCs authorial control over the adventure. If something happens DURING the adventure, I likely already have a good reason for it and don't want my plan second guessed by players. Once you give them control over one thing, then they get in that mindset and start LOOKING for things to change and are quick to make suggestions constantly. Then they also tend to get annoyed with you if you've said yes a couple of times and suddenly say no to one of their suggestions. Then they start to metagame WHY you said no to this particular request and trying to guess at what secret game information they may have gleaned.
It also just slows the game down.
If it's done between sessions, it can at least be done by e-mail and therefore semi-privately and allows for a bit more discussion in terms of saying "Sure, but with a couple of changes to your ideas. How about this?"
Mind control is a very dangerous thing because looked at objectively the players have a lot less information than the characters. Take a film (preferably a good one) and turn on the narration track. Then turn off the screen.
You understand the subtle details of the film based on just that? This is about the level of information even a good GM gives out. Changing the rules on them (as Mind Control does) just makes things a whole lot harder.
Most of the time the narration track doesn't even bother trying to explain what is happening in the movie. It normally consists of "When I was planning this scene, I thought...we need a big explosion. So, we had a crew out there for the better part of a month planning just this one explosion." Which of course tells you absolutely nothing about what happened in the movie other than something exploded.
But yes, I think I can do a good enough job of conveying what is happening through a combination of subtle clues description and intonation combined with throwing in a bit more description when I know something is important.
In fact, it is precisely this lack of detail that lets me steer the adventure in the direction I want it to. If I really don't want them figuring out the answer to a puzzle too early, I can avoid describing certain things unless they ask about them. I can not call for Insight checks unless they specifically ask for them. And so on.
If I think now is the proper time and place to reveal the secret, I can make things extremely obvious.
It's the exact same things books will do in their narration. If they don't want the reader to figure things out, they'll make sure you avoid mentioning it until later.
I don't. In BSG you have an entire cast and their body language to worry about complete with directing and costuming. In a tabletop RPG your information is limited to what the GM can convey and as all the information you get comes through the narrator (i.e. the GM) you must be able to trust that narrator. The only way I see that working is as a moderately large scale LARP or with a group that knows each other very well.
Well, given the writers didn't even figure out who were Cylons until a short while before each of them were revealed as Cylons, I doubt body language or costuming, directing or costuming would have tipped you off. Each of the actors, directors and costumers had no idea who the Cylons were so they couldn't have shown it, even subtly.
Instead, they were just revealed at the appropriate time to make the most impact on the story. Which is still fun in its own way. Yes, it means that it was likely impossible to ever figure out who the Cylons were. But that doesn't make their reveal any less dramatic and interesting. Sometimes a plot twist can be fun simply BECAUSE it is a plot twist.