Dubbing is a bigger part of movie making than you might think. If it's well done, you never know it's happened. Phil Rhodes lifts the veil on this dark art
While we were waiting for a very noisy helicopter to fly by, someone of my acquaintance once described television as “radio with pictures”. At the time, in the middle of a complicated shoot, this seemed ungenerous, but perhaps we're being unfair. Cameras can exclude things with great precision simply by framing them out. Nobody knows, while watching a historical drama, that there's a “no parking” sign barely out of shot. However, a considerably more distant jet aeroplane can make the audio recorded for that scene anachronistically unusable even if it doesn't make the dialogue unintelligible. In short, we don't often consider the fact that while modern microphones can have good rejection of off-axis sound, none of them is nearly as perfect in excluding its surroundings as a camera and a lens.
So, much as it pains me to say it, the sound department is frequently short-changed, and technologically less able to do anything about it. Therefore, call it what you will; looping, ADR, or dubbing is often essential. There are reports of actors and directors who value the opportunity to tweak performances, though few people really relish trying to recreate a performance in sync with the original take. What's more, there can be a terrible lack of opportunity for the actor to prepare and get into the character, especially months after principal photography.
ADR, as an actor recently opined to me, sucks.
Various approaches to timecode synchronisation are possible, especially with software such as the incredibly cost-effective Reaper digital audio workstation which is capable of decoding timecode from a spare audio input and recording dubbed takes onto various tracks. Unfortunately, unless we consider exotic setups which are capable of producing timecoded rushes on set, this really only works with something approaching a locked edit, and many quick-and-dirty configurations designed for near-set use will simply use a laptop to play back the video with the original production audio. In this case, recording wild to a field audio recorder, synchronisation would be dealt with by eye in a nonlinear editor, especially where there's a need to trim, stretch and edit for good overall sync.