Ultimately, the play is less about the boy than Martin, who has his own mini-breakdown thanks to the questions Alan's behaviour throws up.
'Modern society has taken us away from certain powerful roots that Martin feels very heavily,' says Griffiths. 'Alan opens up to Martin the true extremity of the world.'
The play has been described as a 'sicko-psycho' drama in the press, something that prompts Griffiths to launch into one of his famous rants (he's well known for giving people whose mobile phones have disrupted his performances a dressing down).
'It's like that bastard who used the word paedophile when reviewing The History Boys on the BBC,' he fumes.
'It was only so he could hijack the programme. This group of people who chatter professionally for money: they talk about the most life-stultifying rubbish imaginable.
'But of course, using the word sicko-psycho means everyone can stop thinking about what's going on.'
The son of deaf and dumb parents, Griffiths, 59, grew up in Yorkshire and went to drama school partly because he didn't know what else to do. 'They said you don't need to take exams in drama, so I said that'll do.'
Griffiths is intensely frustrating to talk to: every question prompts a long, meandering answer. One about whether he thinks Equus has dated (it's implied in the play that Martin will use electroshock therapy on Alan) results in an anecdote about Ronnie Kray.
Yet he's charming company, a real raconteur. Mid-flight, he breaks off to relate a late-night incident at Tesco.
'This chap asked if I could confirm for his son I was Uncle Vernon [Harry Potter's evil uncle]. I furiously told him to push off: how dare he bother me like that? The man looked crushed but his son chirped up: "Yes, that's him."'
He's only nominally interested in a return to classical theatre - 'really, the revenue is so hateful'.
Despite high-profile appearances in The Naked Gun 2 and a 1/2 and TV series Pie In The Sky, Griffiths is relieved that, thanks to Hector and Vernon, he's thrown off the stranglehold of Uncle Monty.
'The public like to identify you with a character,' he says. 'You need to get them to associate you with several. That way, they realise there's a you in there somewhere, too.'
Equus, opens tonight, booking to Jun 9, Gielgud Theatre, 33 Shaftesbury Avenue W1, tonight 7pm, otherwise Mon to Sat 7.30pm, Wed and Sat mats 2.30pm, £19.50 to £49.50. Tel: 020 7087 7599. Tube: Piccadilly Circus
