Dubbing, processes by which sound and dialogue are sometimes added to a film after it has been shot. Dubbing is most commonly associated with the substitution of one language for another in a film. Generally, actors' voices are recorded while a film is being shot, but in Germany and Italy, for example, almost all foreign-language films shown outside of specialist cinemas are dubbed. In films with large international casts—Luchino Visconti's Il Gattopardo (1963; The Leopard) is an example—some actors will be dubbed for some countries, while others will be dubbed in others. In countries that have highly developed dubbing industries, the synchronization can be extremely skilful, appropriate voices are used, and a good, if curious, relationship to the original is often achieved. In other countries, such as Britain, where dubbing is less common, it is often less convincing: lips and words are frequently imperfectly matched and characterless voices are often used. Again, as the BBC has shown in some of its co-productions, when care is taken the process can be almost unnoticeable.

These, however, are special circumstances, where dubbing has been pre-planned as an integral component of production. Generally speaking, important rhythms and patterns of speech are lost by dubbing from one language to another; dissonances with sound, image, and setting are unavoidable; and the formal integrity of a work is destroyed. Sub-titling, especially for films of artistic value, is, therefore, preferred. Commercially, dubbing is important in allowing a film to appeal to a wider audience, although, except with inferior films, it offers no guarantees and potential audiences can be alienated by it. A film such as Viva Maria! (1965), successful in the rest of the world, “completely failed when it opened in America” because of what director Louis Malle described as “very mediocre dubbing”.

Dubbing is not simply the substitution of sound or dialogue; it has to take place after, not during, filming. The instance, in the famous scene in Blackmail (1929), directed by Alfred Hitchcock, in which the actress Joan Barry's voice was substituted, on the set, for that of Annie Ondra, whose voice was not recorded, is not, therefore, an example of dubbing.

The term “post-synchronization” is sometimes used synonymously with that of “dubbing”. Quite frequently, in scenes where unpolluted sound recording is difficult, perhaps when filming on location, guide tracks, which are replaced later in the studio, are made. The actor listens on headphones to short endless loops of the original recording, and re-records them in technically better conditions. In Italy, the process goes further and even the voices of Italian-speaking actors are frequently dubbed by others, an anachronistic practice that began towards the end of World War II, when sophisticated recording equipment was in short supply. Frequently, the voices of professional singers are substituted for those of actors; the singing of Ava Gardner in the film version of Show Boat (1951), for example, was post-synchronized by Annette Warren. This raises interesting aesthetic questions in relation to acting and directing.

The final dub, or dubbing session, occurs when the work of all the specialist artists associated with the sound are combined together for the final soundtrack of the film, and takes place in a dubbing theatre that contains a console along with all the necessary equipment to mix the various sounds together. The relative balance of sound, music, and dialogue is adjusted, and such elements as echo and reverberation are added. Individual sound effects, such as those made by the movement of clothing or the rustle of leaves, are integrated with the score, the sound, and the dialogue. Great care is given to matching correctly sound and image; for example, each gun has its own distinctive sound quality. The final achievement is an artistic as much as a realistic effect, one that convinces the audience of its authenticity while simultaneously carrying it through the drama.