While Michael and Jermaine's lead vocals were live, the other brothers' background vocals were pre-recorded, as was Debarge's background vocals (There's short footage if you can find it online of Smokey and others in a recording studio recording the J5's background vocals for I Want You Back)

I think other groups' had some pre-recorded vocals also


http://boundless-expression.tumblr.com/

http://themichaeljacksonarchives.tumblr.com/