Juan is correct, inflight reversing was a bit noisy; however, it was a MEL requirement to have the inboard reversers operational to operate above FL350. The procedure was as follows:
For a steep or emergency decent all throttles are retarded to idle and the inboard thrust brake levers are put into "Reverse Detent Thrust" and when the "Eng Thrust Brake" indicating lights come on the throttles are moved to the reverse power stop. The outboard engines will be at flight idle as reverse in engines 1 & 4 is not possible until the landing gear is extended. Additionally, the use of reverse thrust was to be discontinued below 190 knots.
Sorry Juan, no speed brakes on the DC-8, but yes on the DC-9. The spoilers on the DC-8 were used to augment flight control (three outboard on each side) but only when the gear was down. This made handling the aircraft in gusty weather much easier giving a quick roll response. On landing the spoilers(two inboard each side and three outboard) would deploy when armed by a signal from rear wheel spin-up or, as a back-up, by nose gear compression. The crew could also deploy the spoilers manually by pulling the spoiler lever back and up.