Don't interact, do anything nice, when still very depressed, or you plunge back very hard, but find some stimulation, some distraction, maybe a train journey or go for a walk.

Understimulation will make it worse. But if anxiety is high due to stimulation, distraction, just lie down and try to focus on future plans for when you feel better: plans to help you get out of depression or things that were important to you before. Never ever feel guilty: you need and can use your depression to think, plan, reorganise your mind, reorder priorities, solve the BP living puzzle. It's full of paradoxical traps, but can be solved.

If you can without anxiety, watch a film or TV series, go outside or read a book (even if it doesn't really register).