There are those who say that, any time you use, or have to use, voice over on a film, you have failed. And there is quite a bit of truth to that. It's not an absolute rule. But it is a very good rule of thumb. Ideally, you should never use voice over.
For the most part this is true. There are exceptions though.
Of course voice over is usually used pathetically, almost like a crutch.
But there are lots of exceptions and very good reasons why a storyteller would use voiceover.
1. Dishonest protagonist (intentionally or unintentionally)...say your narrative is from a single point of view, first person kind of film. You can create lots of conflict if what the protagonist tells the audience doesn't match up with what they see on film.
2. Someone telling a story within a story...there are lots of examples of this in film, and it's a good technique to dramatically slow down the pace of a film without stopping the pace. Depending on the storyteller's objective, if the camera follows another story within a story, of course it will usually include voiceover that is contained within the original narrative. (or in Forrest Gump, the opposite trick is used)
3. Conflict within the character's mind...this is very tricky to do right. Most films that would even attempt this will fail at it, even professionally. I'm just not sure it's possible to do if you have an omniscient narrator/protagonist; however, if the narrator is NOT omniscient, perhaps he/she voices over, but then the film jumps to real time, etc...perhaps it could work that way.
4. You might use it once or twice in the film only. Typically near the beginning and/or conclusion. A really talented writer might use it once or twice for a point of emphasis or for style or simply for the hell of it.
I'm sure there are other examples. But, for the most part, people use it as a crutch and it's bad storytelling.
It's like a flashback. 99% of the time it's not done well.
Simply, any voiceover should serve a very specific dramatic point (exception for point #4).
It should not be used to just "tell" a story. Even if someone thinks they have some clever twist or something, nobody will care.
Because someone just having a solo conversation is boring. It's not dramatic. It won't work.
