At a Q&A after yesterday's BFI screening of both parts of Henry IV, director Richard Eyre said that the plays were his second favourite in the Shakespeare canon (King Lear being the first) and that he'd undertaken the task of filming them for the BBC's Hollow Crown series on one condition: that Simon Russell Beale played Falstaff.

Russell Beale, sitting alongside Eyre and Sam Mendes, the executive producer on the project, modestly said that ninety per cent of his performance was the make-up and the fat suit. "Mind you," he said, patting his once-again capacious tummy, "I probably wouldn't need the suit if we started again tomorrow."

He had popped into the BFI from a Timon of Athens rehearsal next door at the National and had been gratified to hear the robust laughter that greeted the mix-up over Tom Hiddleston's Prince Hal nicking the crown from the pillow of the not dead yet King Henry of Jeremy Irons and trying it on for size.

It's typical of the Eyre production, though, that the scene is fraught with ambiguity as well as misunderstanding: Hiddleston scarpers with the crown and sits on the throne next door, not in any triumphal way, but in a state of acute apprehension. He suddenly dissolves into tears. Is he sorry for his father, or himself?

An air of mystery, perhaps impenetrability, surrounds so many of the characters in these great plays, something I hadn't fully appreciated before. And none of them are very nice or morally attractive. "Falstaff's a shit," said Simon, and Eyre chimed in with, "and so is Hal, and so is Poins. Hotspur's not a shit." And what's to like about Julie Walters' raddled old brothel keeper of a Mistress Quickly? One minute she's fawning protectively over Falstaff, the next shopping him to the peelers for unpaid bills.

And how about Harry Hotspur? Joe Armstrong, son of Alun Armstrong who plays his dad, Northumberland (confusingly played in Richard II by David Morrissey; what happened to through casting?), is a bundle of random, mis-directed energy, and even starts making out with his own wife (Michelle Dockery) while sniggering at the Welsh singing of Lady Mortimer.

Falstaff, said Simon, was a good example of the unexamined life, a man whose feelings for others were a complete mystery to him. The fact that he's not even sure about his affections for Maxine Peake's raunchy, bum-baring Doll Tearsheet goes a long way to explaining the shocking callousness with which he wounds the already dead Hotspur and claims the corpse as his own battlefield trophy.

The battle scenes are much better in this film than in Thea Sharrock's Henry V, partly because Eyre has made a close study of how Orson Welles dealt with them in Chimes at Midnight and partly because he's found a way of sewing them into the dialogue of the plays. And there's a wonderful shot of Irons trotting slowly through the field at Shrewsbury in the aftermath of war, a snowy landscape littered with bodies, horses and bedraggled remnants of the rebels' regiments.

Kenneth Tynan said that if he had to be in a theatre when he was dying, he'd hope to be watching the recruitment scenes of Henry IV Part 2. The trouble with them is that they are so good, and so funny and touching, that they rarely live up to expectation. Nor do they here, but they're not half bad, and David Bamber's Justice Shallow, one of the most decrepit and revolting little characters in the entire canon, is careful not to shatter too many proprieties. He quivers like a feather and creeps like a slug. His sidekick, Silence, is given a stutter and a great big empty moon face by Tim McMullan, a lovely cameo.

It's always a pleasure to tick off the actors in a large cast epic like this: Geoffrey Palmer is wonderful as the jobsworth Lord Chief Justice, especially when he starts to squirm; Tom Georgeson is a superbly florid Bardolph; John Heffernan a funny, put upon Francis in the tavern; and Matthew Tennyson, so good in the Robert Holman plays recently at the Donmar Warehouse, a watchful Thomas of Clarence, alongside Will Attenborough (Michael's son, making his debut) as his younger brother.

And in other small roles, you can spot Iain Glen (this year's first, and surely best, Uncle Vanya), Dominic Rowan (soon to open in A Doll's House at the Young Vic), Niamh Cusack, David Hayman and the always remarkable-looking Adam Kotz as Lord Hastings.

I'm not at all sure about the music, though. There's far too much of it and it tends to drift into moody anonymity too often. Apart from that, both films are hugely enjoyable and should solve the problem of what to watch on the telly these two coming Saturday nights now that the football's finished in the Ukraine.