Best Way to do this?

I hate to say it, but I think this is a lot more complicated (and difficult) than you might initially imagine. First, you'd actually be dealing with not one but two green screens, as people would presumably be moving both in front of and behind your actor. Either that, or you'll have to do a RIDICULOUS amount of rotoscoping.

And who are the people in fast-motion? Do you have a budget to hire the massive amount of extras needed for this? If not, are you thinking you'll shoot plates in public places? How are you going to control that environment? How are you going to keep the general public from not walking where your actor is to be placed? How are you going to keep the fuzz off your back?

How's this for an alternative? Instead of having all the extras walk in fast-motion, why don't you have them in normal speed, while your main actor is in slo-motion? All of the complicating factors I listed above would be significantly reduced by simply reducing the amount of time you need to record (and therefore the number of extras you need).

I will have to take that advice into thought. But my idea however, there wouldn't be a huge crowd, just 20 people at max, and it would be inside of a building. So I wouldn't have to worry about extra unwanted faces in the shot or anything that would get me caught up into some trouble.

I wasn't talking about you getting into trouble. If you do time-lapse, the faces will be blurred, so you'd be fine. I was talking about how long it takes 20 people to walk past your actor. In real life regular-speed, maybe five seconds? In time-lapse, it's much shorter. I assume you want this shot to last more than a split-second, no?

Why not do it "live"?

It will be difficult on the actor but it can be done. I did it on super8
film many years ago.

I could be wrong but I believe you're overthinking this.

I would just have the guy stand really really still, lock down the camera on a tripod and then have everyone walk by. Then speed it up in post.

If the guy moves too much and it looks unnatural, duplicate the layer, make the top layer super-slo-mo and rotoscope a mask around the guy. BOOM roasted.

^ These two make good points.

That is much easier. Thanks!

Yep.. do it in camera. This technique was used in numerous episodes of Scrubs, and at least one scene in Garden State. Since Zach Braff was involved in both that TV show and that movie, maybe it was his idea. *shrug*

Anyway, in camera, easy peasy.. Especially if your actor has any experience as a live mannequin