Why do files look bad whenever I slow them down? I'm using FCP 5, and it's shot with an XL1s.

Good looking slo mo is shot at a high frame rate (if video = 30fps, 60fps would run at 50% speed)... This catches all of the unique timeslices. When you stretch a 30fps clip out in FCP...it repeats frames which gives a pulsing movement instead of a smooth one.

You can deinterlace a 60i clip and turn it into a 60p clip (I have a script and instructions in this thread: http://www.dvinfo.net/conf/showthread.php?t=68425

You won't be able to see my sample clip currently, but the response seems to be good from folks who have used it...I would recommend you shoot 60i at 1/120 shutter for best results...you can go with a faster shutter for more discrete timeslices (strobing like in the zombie attacks in 28 days later) which will have less motion blur between each frame...more like a moment in time rather than a frame from a movie.

That's amazing. So I need to be shooting with a shutter of at least 1/60? I don't understand scripts. Where/How do I use them?

with the one from the other thread, you double click when you get to that point in the instructions...it'll ask you to select a folder, then do its thing.