Well, I wouldn't say he's serious all the time. I've seen him in funny interviews and he was in the animated movie Penguins of Madagascar (which isn't the same but it is animated and while I haven't seen it I'm sure it could be silly).
Where did you read that it's possible Benedict might be in it? I'd be doubtful at this point until we get more confirmed. I didn't like Frozen that much but if Benedict were in it I'd at least be curious what kind of role he'd be playing and whether he'd be singing anything.
