Every particle you see orbits the central mass on a Keplerian circular orbit: angular velocity ω(r) = √(GM/r³), so ω falls off as r-3/2. Halve the radius and the orbital speed rises by a factor of about 2.8 — this differential rotation is exactly why a real accretion disk shears itself into a flat disk instead of staying a spherical cloud: neighboring rings of gas rub past each other at different speeds.
That shear is friction (viscosity), and friction does two things at once. First, it converts orbital kinetic energy into heat, radiated away as light — which is why standard "thin disk" theory (Shakura–Sunyaev) predicts an effective temperature profile T(r) ∝ r-3/4: the innermost annuli, moving fastest and rubbing hardest, glow far hotter (near-white/blue here) than the cool outer disk (deep red). Raising the central-mass slider deepens the potential well — orbits speed up at every radius (ω ∝ √M) and, for the same viscous heating, the whole profile runs hotter (T ∝ M1/4 in this model), matching the real-world pattern that stellar-mass black hole disks run far hotter than the disks around supermassive ones.
Second, viscosity transports angular momentum outward, which is the only way matter already on a stable circular orbit can ever fall further in: each ring slowly loses angular momentum to the ring outside it and drifts inward with a radial speed that grows the closer it gets to the black hole (v_r ∝ -ν/r in this simplified diffusion model). Matter that crosses the inner edge (the last stable orbit) is swallowed; a fresh particle reappears at the outer edge so the disk stays in the same steady state real accretion disks settle into.