In the giant Rashba semiconductor BiTeI, electronic surface scattering with Lorentzian linewidth is observed that shows a strong dependence on surface-termination and surface-potential shifts. A comparison with the topological insulator Bi2Se3 evidences that surface-confined quantum well states are the origin of these processes. We notice an enhanced quasiparticle dynamics of these states with scattering rates that are comparable to polaronic systems in the collision-dominated regime. The E-g symmetry of the Lorentzian scattering contribution is different from the chiral (RL) symmetry of the corresponding signal in topological insulators, although both systems have spin-split surface states.