@Prish. I would say, as much prep time as you need, really. We're not on any sort of time table (Timelock) here, so everyone can contribute when they're able, and otherwise not. No harm, no foul.
We may want to get down to brass tacks sooner, rather than later, but there's no strict deadline.
I thought of Henry as a potential candidate for IC... though they only have that one meaningful scene together. Is there a fair bit of handing off going on here, or just many places where Henry's influence is felt by Thomas from afar? Hmm...
Edit: I'm also having trouble seeing who the change character is in this film. That would give us a big clue as to who the IC might be. Thomas clearly (in my view) is a Steadfast MC and protagonist... but who changes in the film? Or does anyone?