Filmmakers drawn to the generation-spanning yarn often fail to foresee or sidestep a structural booby-trap that comes with the territory. A few weeks ago, in my review  of Andrea Arnold's Wuthering Heights, I referred to the problem as "transplant casting", so let's stick with that phrase: the passing of one character between two or more different performers, with all the hazards this entails.
In Arnold's case, the shift from the earlier incarnations of Heathcliff and Cathy (played by Solomon Glave and Shannon Beer) to their slightly older selves (James Howson and Kaya Scodelario) caused a rupture from which the last third of the movie struggled to recover. Not that it's impossible to make such a transition without the film suffering. Another of this year's finest British movies, Peter Mullan's Neds , used two actors to portray at different ages its protagonist, a bright schoolboy in 1970s Glasgow who veers dramatically off the rails in adolescence. As the younger model, Gregg Forrest is a gawky and uncorrupted naïf, a lamb thrown to the lions. When Conor McCarron shows up to shepherd the character through his teens, the plausible differences in manner, physique and attitude tell us all we need to know about how the boy has hardened like a scab in the intervening years, despite still having a face that would look more at home staring out of a pram.
Why does the handover work so powerfully in Neds? Well, both actors are equally compelling, and there is synchronicity between their performances, accidental or otherwise. This must owe a great deal to the continuity of Mullan's script and direction. Also, the full horror of John's descent in the second half can only be experienced in the context of our memory of his younger self, and all the early potential that was squeezed out of him by factors economic, social and domestic. The entire subject of the movie is contained within that piece of transplant casting. It had to work, otherwise there would be no film. Into the same category I would place Fred Schepisi's Last Orders, which matches up beautifully the youthful performers (notably JJ Field and Anatol Yusef) with their older equivalents (Michael Caine and Bob Hoskins respectively).
When it goes wrong, it can be one of the most brutally disruptive elements in narrative cinema. I'm no cheerleader for Billy Elliot, but Jamie Bell's performance was miraculous -- whatever euphoria the movie provoked was largely attributable to him. So to cut, in the final scene of that movie, from the young Billy to his adult self (as played by the dancer Adam Cooper) bounding across the stage in Swan Lake, felt positively callous. Put it down to the cinematic inexperience of the director Stephen Daldry, or the kowtowing of Lee Hall's screenplay to the conventions of the rags-to-riches formula, but it was a snub both to Bell and to the audience which had cheered him on over the preceding 90 minutes. The impression was of an athlete denied a victory lap.
If a film solicits our investment in one group of actors, it's hardly surprising if we experience something like separation anxiety when those objects of our affection are snatched abruptly from us. I feel that ache even in a work as accomplished and far-reaching as Sergio Leone's Once Upon a Time in America , in which the sadness of bidding farewell to the enthusiastic younger cast members after 70 minutes or so should by rights be cushioned by the fact that their successors in the roles include the likes of Robert De Niro and James Woods. Hardly amateur hour. And yet the first section weaves such a potent spell that it is always with reluctance that I move on to the remainder of the film: not a reflection of its quality so much as the perils of transplant casting.
One simple if time-consuming answer is to use the same performer at different stages of his or her life. I have already written here  about the forthcoming projects by Michael Winterbottom and Richard Linklater which are being shot over the course of many years (five in Winterbottom's case, more than a decade in Linklater's), thereby removing the need for multiple cast members in the same parts. Lindsay Anderson (who made three films with Malcolm McDowell as Mick Travis: ...If, O Lucky Man! and Brittania Hospital ) and François Truffaut (whose key creation Antoine Doinel , played by Jean-Pierre Léaud, appeared in five films, beginning with The 400 Blows) provide only the most well-known instances of directors dropping in repeatedly on the same character and actor.
My favourite example is one manufactured entirely retrospectively when the footage of a young Terence Stamp from Ken Loach's Poor Cow was recycled as flashback material for the character played by Stamp 30 years later in Steven Soderbergh's The Limey . The only downside is that history may come to regard the earlier film as a beast bred expressly for the purpose of donating its organs. Even this innovative species of transplant casting, it seems, is not without its risks to both donor and recipient.