How different would writing & directing be for animation compared to live action? I would think it would be pretty much the some before and after the animators have done their thing. I've always been kind of surprised how little crossover their seems to be with the two styles.
Except that animation writing and directing is more art-driven. Live-action writing begins with the script, and storyboards/animatics come later and are even optional. Animation writing/plotting often happens mostly in storyboard form, and the artists/animators are the ones driving the process rather than following someone else's lead.
Although the case can be made that many live-action feature-film directors work a similar way, spearheading the process with their ideas of what scenes and set pieces they want to feature, with the scriptwriters simply there to follow their instructions and piece those elements together.
But animation is still a different visual language with different mechanics and details. You have far finer control over every detail of image and motion, yet conversely there's much less room for improvisation and spontaneity. Great care has to be put into designing and creating things that can just be pointed at and photographed in live action. Everything has to be planned out on a much more granular, frame-by-frame level, and everything is made from scratch. It's a very different experience.
Of course, animators can make the transition to live action quite successfully. Tim Burton began as an animator. Brad Bird, I felt, did a great job directing Mission: Impossible -- Ghost Protocol
, his first live-action film, and I noticed ways in which his experience and sensibilities as an animator helped him do interesting things with the film, not just in terms of look but in terms of story details, the progression of actions and gags, things like that. The kind of structural precision that comes from experience in the meticulous field of animation where every last detail has to be worked out in advance.
Plus there's the fact that modern action movies are increasingly becoming part-animated anyway. CG special effects are essentially just cartoons with a photorealistic design aesthetic. So experience directing animation can definitely be useful for directing a CG-heavy film.