Voice actors are not chosen until about halfway through the production process. Some shows, usually heavily funded by record companies, have actors set up when they go to storyboards. The actors who do incidental voices and appear in minor roles are cast by one of the voice actors casting companies.
Unfortunately, for the past few years we have been seeing a wave of low quality, inexperienced, untalented voice actors who are the result of the big voice actor boom that started some years ago. Maybe they are OK at karaoke at a bar but they are absolutely terrible in the studio and require dozens of takes to get a scene right. I've seen recording times double as a result of bad voice acting. Studio time ain't cheap either!
The voice actor boom started mostly because the sponsors and producers of shows wanted to increase the flow of money into the company (A reasonable goal). Showing off the voice actors — especially if they are female and even more so if they are cute — sells soundtracks and shows. As directors and other creative staff members often don't want to appear at events and aren't immediately attractive to fans, the voice actors fill a public relations void. The whole idol craze was going on and everybody was trying to cash in on it any way they could. Pretty soon there were "voice acting schools" showing up and they churned out a lot of people, most of whom couldn't act their way out of a paper bag.
Some directors want to use new people for their shows because they don't want the audience to have preconceptions about the characters from seeing the names of the famous actors playing them. There is a lot of talent out there but most of the really good people come from TV and stage backgrounds rather than musical ones (I agree with these directors very strongly).
(One of the unwritten laws of casting, screening and hiring voice actors is never hire an anime fan. They will try to perform in a preconceived "anime" way which is not what works best. However, some studios don't mind much if fans sit in on voice recording sessions. It can be a lot of fun).

The (Re)Animator: Idol Singers, Pop Bands and Anime

Most pop singers and bands in Japan are "produced" rather than evolving independently through growing popularity and interest. Record companies create an image, a sound and have songs written for the singer/band and they lock them into a 3 year contract, which is the average life-span of a successful pop group.
I was having dinner with a very influential music producer and "idol maker" and asked if his company had anybody good coming up which nobody had heard of yet. "We never have anybody good. If we get people who are too good then the fans won't listen to them. We work in the 'decent' to 'pretty bad' range. That's where the singers and bands who are the most popular live."
They tried reanimating dead bodies and having them sing because it's very difficult for a singing dead body to sound better than a living person. (They apparently have never seen the Anime Expo karaoke contest). The dead bodies kept falling apart and the sight of a rotting corpse in a fluffy yellow dress with a pineapple hat and parts dropping off is unattractive to even the most die-hard (har) idol fan.
(A studio I worked at once bought some of the zombies and said (before we saw them) "Hey, we have some idol singers coming to work here next week. They work really cheap so we're going to get rid of anyone who makes more than they do. You want to take a pay cut so you can hang out here with them?" If I was paid any less I would have been paying the company to let me work there so I didn't take them up on it. Imagine the shock when the guys saw the corpses being rolled in! They had on sailor uniforms and bunnygirl costumes so the guys put up with them until the production manager decided to get rid of them because the companies we were contracting cels from kept calling and complaining that they were finding teeth and ears and lips and such stuck in the painted cels. We took the bodies out in the parking lot, stacked them on some sawhorses and burned them. We kept the bunny ears. It wasn't the same after that so I left).
Some of the most creative commercial music in Japan is produced for anime. TV dramas, variety shows and documentaries often use anime BGMs years after the show has ended.
[Editor's Note: I was watching the low-budget sci-fi movie LEVIATHAN and noticed that the BGM sounded awfully familiar. Fifteen minutes later, I had figured it out — they were using the music from RECORD OF LODOSS WAR. Scary, isn't it?]
Recently, anime companies have been looking to US companies and studios to do recording and sound effects. It is very expensive to work in a recording studio in Japan and the SE Things are getting more international all the time!