In scenes where the boom mic would have to be placed much too far from the actor in order to film the scene, how is the dialogue recorded? Is there some hidden microphone on the actor's clothing or does the actor voice-over in post-production?
Off the top of my head I can think of three options:
Plant microphone: a microphone that's attached to, or hidden behind, an object on the set
(e.g. a Lavalier mic can be used as a plant mic)
ADR / Automated dialogue replacement: the actors re-record their dialogue in post-production
I work in film/TV, and here's how it's usually done in professional shoots. The main actors (pretty much everyone with lines) wear hidden lavalier microphones all day long, in every scene. For the most part they're unseen, even in real life (hidden under shirt collars, on the inside of clothing, etc.). However, the audio quality from lavalier microphones is mediocre, which is why shoots still use boom mics. (The lavalier audio is treated mainly as a backup.) So what do you do for wide shots where you can't squeeze in a boom?
Simple: use the audio from the closeup. Remember that for most scenes, the coverage of the scene in the wide shot is usually the beginning or end of the scene, often covering motion like actors walking up to each other. Most dialogue, in the final edit, is shown in closeup where we can see actors' faces well and see their reactions. That's also the coverage where the audio is of highest quality. So what's done is crews shoot all the dialogue in closeup, even if they know that the first line or two will be shown in wide shot in the final edit, and then the audio from the closeup is overlaid on top of the picture from the wide shot. This has the double benefit of having the audio perfectly match the audio from the rest of the scene (i.e. the closeups) and it's of higher quality. Keep in mind that a shot that's too wide for a boom is also so wide that we can't see the actors' lips clearly, so there's more latitude for using audio from other takes.
For the unusual scene that is covered only in wide shot, crews would also record what's called "wild lines" on site: takes for the sound department only, where the camera isn't rolling. For wild lines the boom mic is positioned directly below the actor's mouth, in the ideal position. It's recorded on site, rather than months later in a studio, for two reasons: to more closely match the actor's recorded performance, and so that any background noise (or "room tone") matches any other audio recorded at that location. In general American or Hollywood-style shoots try to avoid ADR whenever possible, as it doesn't sound as realistic as audio recorded on site.