I've since learned that most movies use a technique called ADR, or Automated Dialogue Replacement. The actor is brought into a soundproof audio studio with exceptional microphones, watch the footage of there scenes, and attempt a sort of reverse-lip-syncing. They speak their lines in sync with the video of themselves speaking on set. It usually takes many tries. Here's an awesome video of a similar recording process of voice actors for animated films.
(Of course, there's a lot that happens after the recording, such as mixing which is perhaps more important.)
Another voice that is just absolutely captivating is that of Benedict Cumberbatch. Much of the strength of Khan in the most recent Star Trek is portrayed through the dialogue and monologues of Cumberbatch. He was also the voice of the dragon, Smaug, in the second Hobbit movie released last Christmas.
I decided to play around a bit after watching The Hobbit: Desolation of Smaug and feeling inspired by the voice that was crafted by the actor and sound engineers. I wondered what I would sound like with a little editing. Here is my "impersonation" of the voice of Smaug.