In the New York Times recently, Caleb Crain examined how Campion created convincing dialogue by going to Keats's own famous letters :-
So the movie Keats does talk the way the real Keats wrote. But does he talk the way the real Keats talked? Like most moviegoers, I expect early-19th-century characters to speak in sentences more carefully and elaborately structured than the ones I usually hear, but my expectation may be an artifact of the recording technology then available.
Bright star, would I were stedfast as thou art-
Not in lone splendour hung aloft the night
And watching, with eternal lids apart,
Like nature's patient, sleepless Eremite,
The moving waters at their priestlike task
Of pure ablution round earth's human shores,
Or gazing on the new soft-fallen mask
Of snow upon the mountains and the moors-
No- yet still stedfast, still unchangeable,
Pillowed upon my fair love's ripening breast,
To feel for ever its soft fall and swell,
Awake for ever in a sweet unrest,
Still, still to hear her tender-taken breath,
And so live ever- or else swoon in death.