Now 71, and after more than four decades working on stage with the likes of the Royal Shakespeare Company and the Royal National Theatre, as well as with film directors such as Roman Polanski and Richard Attenborough, and on the small screen in such classic serials as Secret Army, you would not blame the actor for taking it easy.
As it is, Shelley is about to tackle one of the biggest stage roles for actors of a certain age outside of Shakespeare's King Lear. Yet, as he prepares to play tormented theatrical patriarch James Tyrone in Edinburgh's Royal Lyceum Theatre's new production of Eugene O'Neill's semi-autobiographical epic, Long Day's Journey Into Night, Shelley may be gimlet-eyed in his dissection of the play, but he appears positively laid-back at the prospect.
"It's an incredible journey, this play," he says. "You can do it on the surface, and it is so beautifully written it probably works. Or you can start to get into the emotional murk, and that is where we are just now.
The closer we get to opening, we will have to come up for air, but at the moment, we are finding out all the things that do not work before we find the thing that is right. It is like life."
Given Shelley's elder statesman status, one is tempted to draw parallels between him and Tyrone. For all his levity, it is a notion that has clearly passed through his mind.
"Listen, there are things I say in there," he says, making a clanging noise that implies an epiphany of recognition. "There are bound to be. He is an older man, I am an older man. He is an actor, I am an actor. He has made some terrible mistakes in his life, I have made some terrible mistakes, so there is bound to be. He is a miser, and there are reasons for that, and I hope I am not a miser, but I husband my resources, so you use that."
For all his talk of journeys, Shelley is more than aware he has not given himself an easy ride, even if director Tony Cownie has been working with Shelley and the rest of the cast on cutting O'Neill's mighty text to a manageable length, "or we would all still be here at midnight," as Shelley observes. "But who are we to say that it is over-written? It all just tumbled out of him, and he could not stop."
O'Neill based Tyrone on his own father, writing himself as the youngest son in a family plagued by dysfunction, failure, addiction and loss that was so near the knuckle he left instructions to his publisher that the play was not to be published until some 25 years after his death. With his widow transferring the rights of the play to Yale University, Long Day's Journey appeared in 1956, three years after O'Neill's death.
The Royal Lyceum's new production marks something of a coming home for the play.
While Long Day's Journey premiered in Sweden, followed by a Broadway run, the play's first UK production was in Edinburgh. That came during the 1958 Edinburgh International Festival, when Jose Quintero, who had been in charge of the Broadway production, directed Anthony Quayle as James. Also in the cast were Ian Bannen and Alan Bates as the brothers.
The last time the play was seen in Scotland was a touring production in which David Suchet played Tyrone. If these are big shoes to fill, Shelley is only too aware of the scale of the task he is facing.
"When I was offered it I wondered whether I could learn it all," he says. "You get to a point where actors have to face themselves. I have seen it happen in other productions when I have been younger, and older actors have terrible trouble with the lines. How does he know when is the time to stop is quite a question, but the reason I accepted this, once you have read the play, if you are still an actor, you accept.
"If I had said no, it cannot be because of the play. It cannot be because of the part, never mind the money. This is rep. It could only be fear, and fear is something actors face from their youth. But being an older actor, one of the things is just being able to hold the lines and keep it going.
"I had to challenge myself is what I'm saying. A couple of lines in a film, that's fine, but if you're offered something like this, you don't say no. You are only offered a handful of parts like this in an entire lifetime."
For a couple of years, Shelley was most familiar from what he calls a semi-regular role on lunchtime medical soap Doctors. Shelley discovered that show's mass appeal while on tour with Mike Bartlett's contemporary version of Greek tragedy, Medea, which visited the Citizens Theatre, Glasgow, in 2012, when the stage door was besieged by autograph hunters.
There is the matter, too, of the YouTube collage of tender scenes between Shelley's character Jed and his on-screen daughter Zara played by Elisabeth Dermot Walsh, which a fan has posted and set to a schmaltzy soundtrack.
This is all a far cry from Shelley's roots in Leeds, where he resisted becoming an actor until long after his elder brother had, and he ended up at he Royal Academy Of Dramatic Art, London. After seasons in rep, Shelley wound up being cast opposite Sir Ralph Richardson in the West End, and appeared in Richard Attenborough's film of Joan Littlewood's Oh! What A Lovely War. Shelley went on to appear as Donalbain in Roman Polanski's 1971 film of Macbeth, dividing his time between stage and screen in a way that few actors of his generation manage.
Shelley played the title role in Julius Caesar at Shakespeare's Globe, and after performing in Tom Stoppard's play Arcadia in the West End, was visited backstage by Hollywood couple Paul Newman and Joanne Woodward.
Shelley has had a long relationship with the Orange Tree in Richmond, and has worked at the Donmar Warehouse and with a younger generation of directors such as Rupert Goold. Unlike James in Long Day's Journey Into Night, Shelley has kept moving, with little or no chance of falling into a rut.
"Some things I have done well," he says, "and some I have done not so well, but what else am I going to do? I am fit enough, mentally and physically, to keep going, but that will go soon enough."
He added: "At the moment it is the challenge of doing great parts like this. That is what keeps me going."
Long Day's Journey Into Night, Royal Lyceum Theatre, Edinburgh, January 17- February 8 www.lyceum.org.uk