Why is it that in animated movie the person singing isn't the one that talks?

Substituting an accomplished singer for the lead character (voice over) is an established Hollywood maneuver. They have done this with real people as well as animated characters for over 50 years.

It’s often done, but not always. Most of the voice actors in Nightmare Before Christmas sang their own songs (Danny Elfman stepped in for Jack.) Jeremy Irons lost his voice singing “Be Prepared” in The Lion King and Jim Cummings had to fill in for the last bar of the song (from you “You won’t get a sniff without me” onwards.)

There was really no reason why Lea Salonga should not have been both the speaking and singing voices for Jasmine and Mulan. She’s a brilliant talent.

I suppose because the best singers are not the best actors, and vice-versa.

I would have to agree! I mean in my opinion Sharri Stoner did a great job as the voice and singer as Belle in Beauty and the Beast, as well as Jodi Benson as Ariel in The Little Mermaid!

For Who Framed Roger Rabbit?, my guess is that they wanted a deep, throaty-voice like Kathleen Turner for speaking parts, and Amy Irving’s ability to sing in a still sexual, yet higher pitched voice for singing.

It’s not everyday you find someone who’s a great voice actor AND an awesome singer. Some people who’re awesome at singing make terrible voice actors. Like, really bad. It takes some serious acting skill too pull off believable emotion in voice acting.