Was Michael Fassbender actually under the head for most of the movie?

I follow Jon Ronson (the writer of "Frank") on Twitter, and he has answered that question a few times, reporting that Fassbender did wear the mask all the time. There is a Q&A with the cast of the movie on the Collider website, and Michael Fassbender gives more details.

FASSBENDER: Yeah, I found it really liberating. The only thing that it took a bit of getting used to – or never really getting used to – was when we were doing the performances, the sound in there. You know, I would go away and I was learning an accent and I’m practicing it like this and then once you put the head on, there’s a reverb thing that’s going on in there and it kind of puts the hearing off a little bit, and it’s hard to sort of locate some stuff, but that was it.

The rest was a lot of fun. It was really cool. It was like, as soon as I put the head on it gave me an element of mischief. It gives it sort of the anarchic quality of Chris Sievey’s original Frank Sidebottom character. I think [he] really sort of came out in that.

And also, it’s exactly what Lenny was saying. For the other actors, it never mattered that I couldn’t make eye contact really with other actors because Frank is Frank and Frank lives in his own universe, and so I was hoping for them, it would be the same. Is he listening? Is he not listening? Did he hear that? They’re used to him anyway. Except for Domhnall’s character, which is a new introduction, everybody’s used to Frank with the head on. ‘Oh, that’s just Frank doing his thing,’ or whatever it is. Whether I was engaged or not engaged, both times I think it was a help for me and hopefully for the other guys that were responding to him.

And later, from the same interview:

FASSBENDER: It was hot and sweaty in there sometimes. If you’re running around in there, you can’t breathe as easy as you would without it, but like I say, I enjoyed it. I enjoyed having the head on. I actually wanted to bring it on to my next job. They wouldn’t let me. [Laughs]

Although, they did have a second head for the stuntman. The only time Fassbender wasn't wearing it.

GLEESON: There’s another one for the stuntman that had little bits cut out. The one time it’s not Michael is when the guy gets knocked over by the car.