Making objects move by themselves

[Azmyth]

I need to make a small object move like its being moved by some otherwordly force? How do they normally do this? Fishing line? In HD it would be visible I would think.. or would you need to get a clean plate of the scene... and then film the same scene with the fishing line.. and then matte it out?

Wait.. I may have just answered my own question lol.

[Mark Harris]

I think you can remove the line in post. AE is pretty good with this, I think. I think it has some tools for wire-removal. Or go ooooold school and light it and shoot at such and angle that you can't see the line. This is how they did it for ages.

Or shoot the table without the object, shoot the object on green at the same angle, and composite it in...

[clang]

If it doesn't need to interact with other objects, then compositing it in post is the easiest option - watch out for shadows though.

[Jisgren]

Is there anything else in the frame such as actors? If not then you can use stop motion to move the object.

[lowercase]

depending on what the object is and how thick the table is, magnets can be very effective and save you time in post.

[mörkö]

You can also move it by shooting it with compressed air from outside the frame. Just a normal compressor and blowgun/air pistol you can find in any workshop. You get some randomness and character to the movement which might or might not be good.

For one project I attached a small aquarium air tube to the blowgun nozzle and hid the other end near the object for some reason, maybe to prevent the effect on nearby objects but in this case take it easy with the pistol as they might not be designed to handle the resistance of the tube.

Sound, smoke/dust in the air and other lightweight objects nearby might make this problematic.

[Michael Carter]

AE does good wire removal -if there's any camera motion HAVE TRACKING POINTS IN THE SCENE THOUGH. I've torn my hair out over shots where I thought "that corner will work fine"... and it didn't. they don't need to be specific trackers - just make sure there are a couple things in the scene with a sharp contrasty point, in decent focus, that won't motion blur out.

I'd strongly suggest you do the clean-plate thing; AE will have to clone in something for wire removal and that can show... I'd avoid cloning and go with roto in most cases. You should be able to shoot it several ways though.

Stop motion... ehhh. It usually looks jerky, it's very time consuming and usually means a separate camera rig. You can add specific motion blurs in post, but that gets kind of render-ey.

My old photoshop mantra was "nothing looks as real as reality" - find every possible way to use "real" objects, motion, textures, etc in a shot where possible.

[bwwd]

This can be done in many different ways,lines,magnets,somethng on wheels under object and shot under certain angle.It really depends what is the framing and whats the object.
Dont listen to some CGI or stop motion ways are you guys serious ? This is easy stuff ,no need to overcomplicate,its like calling SWAT or whole police because youve heard someones cursing.