You cannot do it convincingly without a lot of pre-production planning and careful shooting. Camera angles and perspectives will not match without it. You can fake it by rotoscoping person from one video and trying to overlay them on another. I would not count on too much success unless you shoot the footage yourself and carefully match camera angles and distance from the camera.
Rotoscoping is a time-consuming, detail-oriented process. Here's a video that introduces the concept in the context of After Effects.
After Effects CC tutorial: Rotoscoping with paths | lynda.com - YouTube
i selected the person i wanted from one video. But then do i replace that
person. it seems confusing. in second video i want all people standing
around a person but i want to replace the person from first video to person
thats in middle in second video. I hope you can help. i tried but i can't
Save yourself a lot of grief. Go re-shoot your scene. You'll spend a LOT more time trying to fix it than you will re-shooting it! Think about this:
- It's not easy to do, even for a seasoned professional -- think about spending many hours or perhaps a couple of days, depending on the shot.
- You're no seasoned professional.
- You'll have to spend many hours (no kidding) learning the process and getting comfortable with it.
- Then you have to actually do it, and it will take you a LOT longer than it would take a professional.
Do yourself a favor. Re-shoot.
If this process is confusing to you it's because you're getting too far ahead of yourself. A baby learns to crawl before it learns to walk. And what you want to do is learn to run a marathon...
Start here: Getting started with After Effects
Be aware that it may take you weeks (or months) to get your ahead around how to work in After Effects. It's a complicated (and very powerful) piece of software.
Or just reshoot.