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camera Timelapse with Regular Speed at Same Time

I hope this is the right section. New to filming, I'm trying to shoot a simple music video and my idea was to shoot a bench on a street in timelapse. Then I wanted to leave the camera set up in the same spot and film the artist performing on the bench. My idea was to have the background timelapse going fast while the artist is performing in regular speed at the same time. Can I achieve this by simply layering the images over each and playing with opacity? Or is that going to look bad? Is my only other option doing a green screen behind the bench?
If you set up the shot carefully, it should be relatively easy and effective. You need your artist to perform in front of a background that doesn't have any (significant) environmental movement, e.g. a wall/corner/shopfront; and then have your frame also contain areas of busy movement (road traffic, pedestrians, clouds [in the sky above the wall]. If the location (and light) is suitable, you could also shoot the artist early in the morning when there was no human activity, record the timelapse later in the day and use a rotoscope technique to blend the two images.

The biggest challenge you'll face will be trying to avoid a dull "static/tripod" shot of the artist, so you may need to shoot several different angles, in which case you'll have to put considerable effort into the framing of each one. It might be worth your while studying how live-action is/was composited with glass matte paintings, not so much for the technical side but for how a huge static image was "brought to life" with relatively little live-action movement.
Couldn't you do this easily by having the actor perform in front of a green screen at normal speed, then put them in front of the time lapse background?

If you do it on location, you will be there for a long time, plus you will need the conditions to look the same for both takes. One for the timelapse, and one for the artist.

At least I think so. I could be wrong, as I've only done timelapse twice.
Hey! I LOVE this effect. Back when I was directing music videos, I did one for a band called Cold War Kids doing this type of double timelapses/real filming side by side and layering them. I had the luxury of having 2 or 3 cameras, but you could totally do it with one if you have some control over the subject and space (I had to mine live, during a concert). I went nuts with the colors and opacity:

Cold War Kids, “ Hot Coals”
Cold War Kids, “Hot Coals”

If you can get a second DSLR for the timelapses and just put the cameras right next to each other, you might be able to get away with it pretty easy (depending on your aesthetics).