This trick comes courtesy of Mike Russell, whose YouTube channel is packed with all sorts of helpful audio post production tutorials.

Obviously, the major caveat with this technique is that it is mostly only applicable to voiceovers and dialogue where the character speaking is off screen or whose mouth can't be seen by the audience -- say the character whose shoulder we peer over in an OTS shot. It could potentially be used to clean up on-screen dialogue in certain instances where the actor's performance is consistent from take to take, but you'd have to be extremely careful not to throw things out of sync.

With that exception -- and it's definitely a major exception -- you can use this technique to really fine-tune your voiceovers/off-screen dialogue and dial in the perfect pacing and word delivery for audio performances.

Source: Mike Russell