You'll play as more than one character
Three, in fact. That's Old Snake, for the first chapter, and Young Snake for the rest of the game. Except, that is, for the second part immediately after Old Snake dies during the middle-east op. Because that's when you'll play as... Raiden!
We see it working out like this. After Old Snake dies, the Foxhound team (led by Meryl) take his body and leave the middle-east. When, as heard in an early trailer, Raiden says "It's my turn to protect you," he's actually talking to Snake's cooling corpse.
Of course, this is Raiden in super-ninja form, so it'll be an all-action episode. You'll travel to South America - in pursuit of Vamp - and leap and bound through some exhilerating conflict with the mooing Gekko Gears. Raiden will finally 'kill' Vamp, though being mortally wounded in the process.
Vamp's body, through tissue grafting (like Liquid's arm on Ocelot) or simple blood transfusion, will revive Snake's dead one. Young Snake will then take on the mission.
Someone will get naked. Or wet themselves. Or be sexually harassed. Or...
We've seen Otacon pee himself while hiding in a locker, Raiden wake up stark bollock naked, and (by using the Raikov mask) Naked Snake triggering some bisexual confusion while being tortured by Soviet nutcase Volgin.
Kojima loves contrasting the oh-so-serious nature of his games with toilet humour or adolescent thrills - think interrupting Meryl as she changes in MGS, the porn mags and FHM posters and EVA's ever-present cleavage in MGS3. So we're surely in store for some silly fun with MGS4.
After all, we've already got a cola-drinking monkey wearing pants. Not to mention the HD-quality gentleman's mags that Snake carries around. Whatever will Kojima think of next?