If you animated for 24 fps, but played back at 12 fps, things indeed would look a bit slower. You just have to be careful, since, at it's core, slow-motion really just looks like slow (very, very slow) movements. - Regardless of what frame rate you're animating at/playing back at.
If you film video at 60 fps and play back at 60 fps, things won't look "slower," although things like motion blur will be noticeably absent. Filming video at 60 fps and playing back at 12 fps, on the other hand - would look a lot slower, since you're playing back at less that you filmed.
It sounds a bit backwards, but then again, so is the process of animation when you think about it.