Has anyone experienced the transition from writing plots/stories for comics to writing a script viable for films? If so, what are the main differences and challenges?
Comic scripts usually include dialogue and location/background descriptions, maybe some directions for character placement. Movie scripts focus more on these details, and add elements of staging, camera placement and angles, and may have guidelines for post-production sound and visual effects. The biggest challenge is thinking of how to visualize how a scene will play out in a film, and what cannot be taken for granted (like sounds or surreal elements).
It's been a while since I've looked into comic scrips, but here's what I remember. There are a lot of differences between a script for a Movie/TV, a stage play and a comic. The biggest one is that in a comic there isn't a director (though the author or artist can fill that role depending on how you work together).
In a movie the majority of the camera work and shots are set up by the director. As a rule you wouldn't have a close up or a wide shot listed in the script because, as the writer, that's not your job. That's not to say you can't do it when it's needed for the story but that's outside the scope of this question.
So, when you are writing a comic script you need to think more about the layout and the arrangement of the characters and the scene. How much of this work you do depends on the artist you're working with. If the artist is really good with that sort of thing it might be better to leave it a bit vague and let them work it out.
Comics have other issues that you will need to think about. The first two are thing things that really make a comic a comic, panels and word balloons.
The standard comic book usually has 9 panels on a page, though that can change depending on what you are doing. What you write needs to fit in that limitation. Also keep in mind that the gaps between panels usually allow for time to pass (even if it's just a moment), which is what makes comics sequential art. Read Scott McCloud's Understanding Comics for an in depth look at this. Because of this your scenes would be broken down into both pages and panels.
Word balloons are a surprising limitation. The normal balloon can hold maybe 25-30 words and you might be able to get away with 3 or so per a panel. This limits the space you have for people to say something, and lot of dialog on one panel can make it feel cramped. This makes for an interesting balance between space and verbosity.
There are also things like caption/narration boxes and the sound effects, but those work pretty much as you expect them to.