What I've found really helpful (but perhaps a bit weird) is to imagine that my antagonist has to complete a SMART goals worksheet, like the one pictured. In the current book, Aegyir, a soul-and-life-stealing entity, wants to destroy everyone, especially anyone who would be able to 'kill' him. Unfortunately, the pesky protagonists don't want him to kill everyone (and especially not them) and keep trying to thwart him.
So, how is he intending to achieve his goal? Why is it important to him? What steps are there on the way to completing this goal (and what issues might stand in his way)?
In all honesty, I don't fill it in in huge detail, but I do think about everything from the antagonist's perspective. It helps me to remember that in the antagonist's world, he's the hero.
In the sheet shown, the key bits are perhaps not so much the SMART goals (section 3), but some of the the other sections:
What is the goal? (Kill everyone, especially the Guardians)It helps me to keep my focus on what the antagonist is trying to do (and therefore where there'll be tension and action between the protagonists and the antagonist). On top of that, it keeps my antagonist having understandable goals (or at least, understandable to them). I always prefer it when the antagonist has a lot of grey areas around them (anyone else think Thanos had a sensible goal, in "Avengers: Infinity War"? Always prefer it when nothing is entirely black or white). Equally, I like my protagonists to be flawed too.
Why is the goal important? (I want to be able to 'live' and not be trapped as a non-physical entity forever; only the Guardians can trap me again, therefore kill all the Guardians)
List potential problems that might prevent you from completing your goal. (The Guardians might trap me; they might destroy me. Basically, lots of things the Guardians might do!)
Action items: (get powerful enough to kill the Guardians by killing others first; to do this, I need to get myself into close contact with the others and away from the Guardians, by ensuring the others and the Guardians are at loggerheads; get them at loggerheads by invading their minds and stirring up hatred...etc. etc.)
What do you like to see in your heroes? Or in your antagonists? Do you like it when they all have shades of grey? Do you like it when you can also root for the antagonist a bit? Or do you prefer it more black or white?