Laser Directed Energy Deposition of thermoplastic polymers (DED-LB/P) offers significant potential for repair and functionalization of components. Polyamide 12 (PA12) and Polyetheretherketone (PEEK) are promising coating materials due to their corrosion resistance, wear resistance, and biocompatibility. However, detailed in-situ analysis of polymer‑specific process regimes are missing, creating a knowledge gap between process parameters and coating quality. This study investigates the process behavior during PA12 and PEEK deposition on stainless steel 316L substrates using high‑speed imaging at 15,000 fps. A thulium fiber laser (λ = 1,940 nm) was employed to exploit characteristic polymer absorption bands. Energy densities from 1.39 J/mm2 to 3.98 J/mm2 were investigated on smooth and sandblasted substrate surfaces. High-speed observations reveal that particles do not melt during flight but soften and bond exclusively upon substrate contact, suggesting substrate contact as a necessary condition for particle bonding. Distinct process regimes are identified for PA12 from minimal adhesion through melt pool formation to thermal decomposition. PA12 undergoes self-limiting intramolecular ring closure releasing volatile laurolactam fragments, enabling a wide and controllable process window. Sandblasted substrates shift the onset of stable melt pool formation to lower energy densities through enhanced energy coupling, extending the effective process window. Deposited volume and surface roughness Ra independently confirm the identified regime boundaries. In contrast, PEEK exhibits an extremely narrow process window, transitioning abruptly from particle adhesion to explosive thermal decomposition without melt pool formation. This behavior is attributed to PEEK's radical chain scission with concurrent exothermic char oxidation, preventing stable melt pool formation.