There is nothing easy or fast about hand rotoscoping. If the shoes are the same color or contrast as the pavement the Rotobrush wil have difficulty discrimating pixels.
Separating a foreground object, such as an actor, from a background is a crucial step in many visual effects and compositing workflows. When you’ve created a matte that isolates an object, you can replace the background, selectively apply effects to the foreground, and much more.
Conventionally, segmentation of a moving image into foreground and background elements has been accomplished through rotoscoping—defining mattes by manually drawing Bezier curves (masks) on most frames, with some interpolation. (See Rotoscoping introduction and resources.)
The Roto Brush tool provides an alternative, faster workflow for this segmentation and creation of a matte.
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> I've seen it done, but i'm sure they wouldnt have animated masks to do it...
Don't be sure of that. Conventional rotoscoping by animating masks frame by frame is very common. It's tedious, but it's also very often necessary. (It's the main reason that the credits are so long for visual-effects-heavy movies---armies of rotoscopers.) Even if you use the Roto Brush tool, as bogiesan mentioned, you often have to clean up parts with manual animation of masks.
See this page for information on rotoscoping.
Start here for Roto Brush information.
This page gives an overview aof all of the ways to do compositing in After Effects.