Sebastian....darling~.
….. — Jim?

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Sebastian....darling~.
….. — Jim?
✉
[SMS: Boss] Oh? And what else do you want to do to me, boss? SM
consultcriminal replied to your photo: LOOK HOW NICE AND ORGANIZED IT IS THESE ARE JUST...
//………….Madam, my firstborn’s soul for your gif folder.
One thing is certain. If you manage to get me in a suit, I'll make damn sure you'll be ripping it off me before long.
Headcanon Two: Anger
Sebastian has a violent and unpredictable temper even at the best of times. It was once joked in the army that he has an on/off switch for his temper. There is no middle ground for Sebastian, no lead up to the explosive outburst, no slow burn, just fast and explosive anger. Sebastian is like thermite. The second someone lights the fuse he goes off, burning with such heat that nothing remains.
Nothing can calm him down, cool him off.
Not even Jim can bring him down when he reaches a certain point, not when his whole being is so lost to the fury, not when the anger races and burns through his bloodstream.
But just like it was joked, the second he hits the peak his temper starts to drop and that's when Jim can calm him. From there the switch can be flicked, he can easily regain control once more. The anger may still bubble under the surface, Sebastian may still threaten and curse but he won't do anything. Before Jim he would have if provoked, the switch would flick back and he would be off once more, but Jim manages to hold the switch down inside of him until he is utterly rational once more.
In smaller situations a hand on his arm and a quiet "Bastian', calm" from Jim would calm him, but sometimes it takes more. Its rare Jim has to result to using any form of authority over him but once and man of the army; you are always a man of the army and will always react to a firm voice. Jim has this mastered to a fine art.
In all, Sebastian's anger is a force to be reckoned with. After all, you don't just get the nickname Tiger due to a few scars.
Hannah's Official Guide to Writing Sebastian Moran in the BBC!verse.
The entirety of what we know about him is either ACD canon, headcanon/fanon, or from Kim Newman's book.
This means that Sebastian is entirely up to you. He becomes your character when you write him next to a BBC!Jim.
It's okay to incorporate established fanon (I sure have), but I think it's more interesting when I read a Sebastian who isn't exactly who I now expect him to be. You can make him stand out from fanon!Seb.
Honestly you really can't make a mistake. Some of the BBC characters have been seriously heavily adapted (Jim for example). Use your imagination. Create a character who plays well off of your Jim. I don't read mormor for how bloody and badass Sebastian Moran is, or however many cigarettes he smokes. I read it for the relationship between him and Jim, whatever that might be.
Everyone's Sebastian is different, and yours will be too.
Other thoughts on Sebastian's character (seriously I could talk about this asshole for hours)?