What makes for excellent voice acting?

[MurrayL]

I've been playing The Longest Journey lately (this is actually my first play-through!) and I've noticed a lot of people saying it has 'amazing' or 'excellent' voice acting, and that the voice-over work is one of the key things that make it such a great game. What I haven't seen, though, is anyone explaining why it's so excellent. No-one goes into any detail about what makes it brilliant, they just say that it is.

We're shooting for a fully-voiced release of Astroloco: Worst Contact when it comes out (see our In Development thread here), and I want to know what you think makes voice-acting great.

If you've done VO for adventure games before, what are the major pitfalls, and things to aim for?

[CaptainD]

That's not really a particularly easy question to answer, but I think I'd have to say the key to it would be that you can actually forgetting that it's voice acting, and just feel like the characters are naturally talking.  I think TellTale's Sam and Max games in particular are terrific examples of this - once I've seen the characters and heard the voices, I just can't imagine any other voice for those characters.

When voice acting goes wrong, it's usually a case of poor translation or poor emphasis.  In some games it actually sounds like someone is reading awkwardly from a prompt sheet without any real understand of either the words used or the way the character who's talking feels.  To me the Secret Files games (at least the English versions) were particularly noticeable examples of bad voice acting.

I don't know whether that's partly answered your question - probably not really, but it's the best I could come up with!

[Crimson Wizard]

I am well acquainted with numerous native localizations of European and American games, and voice acting is done simply terribly in my opinion (translation too sometimes, but that's other question).
Main problem those actors having is usually that (as it seems) they are focusing on "professional" diction, rather than emotion and actually playing the role. Lots of pathos and no actual character. Sometimes it sounds like they are having fun reading those "silly" lines, or maybe feel awkward because of dubbing computer game, I dunno, and hence you can literally feel the irony in their voices.
The result is instant loss of immersion.

E: Looks like I answered "what makes voice-acting" terrible instead