I watched this last night and enjoyed it a lot, but don't expect some rousing Arthurian adventure flick with sword fights and jousting action. This is a contemplative, atmospheric, sometimes sinister film which you probably get more out of if you know a little about the Arthurian stories in general and Sir Gawain and the Green Knight in particular. I knew some but brushed up via Wikipedia, and it helped. The film sticks to the basics of the story but also deviates in ways which reframe ideas about heroism and knightly virtues. Dev Patel is very good in the lead but he is supposed to be quite a passive character, a layabout whose mother here is implied to be Morgan le Fay, who lights a fire under his lazy ass by summoning the title character. King Arthur and Queen Guinivere (Sean Harris & Kate Dickie, nicely cast against type) are middle aged, tired and sickly looking, characters whose time has passed rather than the romantic take of previous adaptations.
David Lowry is becoming one of the most interesting US filmmakers, he started out with the Terrence Malick influenced Ain't Them Bodies Saints but has moved on from that, though there still is something ethereal about his films. A Ghost Story is one of my favourite films of the last decade and this has a similarly melancholy quality. The Green Knight is far from a Hollywood blockbuster, a slow, mysterious, often ambiguous film and its my favourite film of the year so far. It's an A24 production, who have specialised in art house horror films like Midsommar and The Witch and that what this often feels like.