Saw footage of on location shooting on youtube for the new Sherlock series (no idea what the scene is); the wind was so loud, it's amazing the actors were able to hear each other at all. Those shoots will obviously be voice-over recorded. Does this happen with every scene? Or just some of the scenes? Like if they are filming an episode, do they have to act it out twice - second time with voice over. On the upside, they don't have to worry if they screw up their lines the first time coz they know their voice will be taped over again later.