If you’re in a sad mood and you’ve got a sad scene to do when better to write it? You don’t have to write in strict chronological order. Conversely, use writing to help improve your mood. Being creative in itself is a positive thing. If what you produce is good, even better. (And if not, it’s on its way to being better because a darned good edit works wonders! And everybody needs said darned good edit on whatever they write before sending it anywhere. Even Shakespeare didn't get it right first go).
Show your characters’ moods well and how they change.
After all we’re not in one state of mind all the time so neither should they be. Moods affect actions affect consequences and plot! Show your characters learning from their experiences - what not to do again for example - and where appropriate where a character refuses to learn. Look at why they have that refusal - is it just stubbornness or are they afraid of change? Have they good reasons to be afraid?
Show your characters’ expressions.
I tend to get Eileen to grimace a lot (!) (though to be fair she has cause) and am aware I need to vary her expressions. Also show your characters trying to hide what they really feel - after all we do it so why shouldn’t your creations? What happens when a character fails to hide how they feel or shows their emotions to the wrong person? What catastrophes could be unleashed?
What calms your characters when agitated? What cheers them? What depresses them? What do they get angry about?
If you know the answers to the above, you know your characters well. Whilst I don’t think you can separate character from plot, both are essential for a good story, a good plot with a weak character will fail as will a good character with not “enough to do” or in the wrong storyline for them. You can only know what story line is right by knowing the character well enough and knowing what they would/would not do in any given circumstance. Ask yourself if you're stretching your character well enough too.