Slow effect with words matching

Hello i need help with a video i am going to be doing its a music video, but i wanted everything slow motion but the words of the song still matching the artist lips while his singing, and also is there a way i can make only one person slow motion instead of the whole video?

Do you have an example of this we can see?

If I understand correctly, there are several things you can do. Either film the singer on green screen. That would let you superimpose the singer over the normal speed video. But speed up the song first, play back at that fast speed, have the performer lip-sync/act the song. Film it at 60fps. Then in your FCPX timeline, make it 24fps, and slow him back down to normal playback speed to match the background video clips.

Or speed up the song, have the singer perform to it as you shoot, then slow it down in your Project timeline in FCPX. That would effect everything in the frame.

So if I shoot it in 60fps how fast should I make the song?

40% faster, because 60fps conformed to a 24fps timeline, slows it down about 40%.

Okay also I plan on slowing the footage down to maybe 50% or 25% also with the optical flow feature in fcpx should still speed it up to 40% ? Or 50% or 25%

I suck at math...

Tried it out thanks to youre information it worked perfectly!