If you are trying to get someone to be in your film without their consent, I would advise you to just go and talk to them (but I don't think that's what you're asking).
If you're trying to be inconspicuous while filming to get a street journalism or anthropological type of footage, then I would recommend using a small camera with interchangeable lenses, such as a Canon EOS-M or Pentax-Q, and use a lens with autofocus.
Turn off the AF light or you might get spotted. Shoot from the hip, because most people will look at your eyes. Wait for people to be looking at something else. Pretend to be doing something else. For the sound, you could be wearing the microphones on you under your clothes or somewhere hidden. You could also have the camera around your neck/shoulder rolling and pretend it's not, like let it dangle as you walk and then stop somewhere pretend to look at a book or a map and fine tune the focus and framing using an app on your phone that recopies the camera's screen.
One trick that really works is having the camera on a tripod and having a couple of friends with you, and standing around the camera and chatting while the camera is recording. If someone comes to ask you if it's rolling, you can say no, you're waiting someone to start. In this case you could be using larger cameras and sound recording devices, but it's a little harder to set up and you can't really do it twice in the same place.