Not sure what your asking here, PP. Are you wanting to write an
imaginary conversation between Charles Darwin and God, and then
take him on from there aboard the Beagle and show how his encounter
affected the developemnet of his theories - either ending up
exactly as they did, regardless, or in your fictional account,
developing along rather different lines?
How you wrote such a scenario would depend very much on how you
chose to view God. I think you'd have to decide from the start
(without necessarily overtly revealing to your readers) whether the
God Darwin encounters is, in fact, the Creator of the universe, or
some other conglomerate distortion drawn from man's immaginations
through the centuries. That is, whether he actually is God, or just
some mythical being.
If the latter, you face one set of problems. How do you converse
with a myth? Is the conversation all inside his own head? If so, is
his reasoning going to be much different, if at all, from what we
already know of his 'evolutionary journey'?
Because of that, you may decide it's got he be the former. It
really is the Creator of the universe he's talking to. A different
set of problems. If he goes off on his journey of discovery already
'knowing the answer', as it were, how could he ever have developed
his theory? Perhaps he still could, if he began to doubt his God
encounter in the light of what he thinks is evidence to the
contrary. That could be an interesting angle - that in spite of God
granting him direct access to hear the truth, he still allowed the
athesitic doctrines of his peers to influence his thinking about
interpreting the evidence he was discovering.
Or, as I said earlier, your ficticious version could show how his
God encounter enabled him to put a quite different (and equally
valid) interpretation on the evidence he collected on his Beagle
journey. You've certainly got plenty of scope here. Good luck with
it. Write on, PP.