Creative process revealed in National Gallery's fascinating Michelangelo, Sebastiano show

I approached the National Gallery's new lead exhibition with skepticism. Focusing on a partnership between Michelangelo and a little-known colleague, with many more of the latter's works that the former ... and including copies rather than the real thing? Were the curators getting desperate? Would nobody loan them anything better?

I was wrong. Michelangelo and Sebastiano is a fascinating show that shines a light onto how the collaborative process actually works. We spend more time on the painters as individuals, rather than just critically regarding the art they left behind.

And those copies? Absolutely brilliant. There's an exact plaster cast of Michelangelo's most famous Pieta, liberating that great work of art from its usual distant place enclosed in a perspex diorama well above your natural line of sight. Here, you can wander all around it, getting up close to appreciate the drama and skill of the composition. The reproduction of the Borgherini Chapel is a wonder of modern technology, as the highest tech digital scanning and printing give us a copy so perfect it even replicates the battered power outlet some cretin drilled through the fresco in the last century.

There's no denying that Sebastiano del Piombo was a lesser artist, and no surprise that he's not very well known. While you wouldn't turn down a gift of one of his works, they're hard to distinguish from an army of candy-pastel coloured saints, Tuscan landscapes and fat putti rolling out of workshops up and down the Italian peninsula in the 16th century. He was, however, in the right place at the right time.

In the early 1510s, despite the fact that he'd already painted the Sistine Chapel and the new pope was a childhood friend, Michelangelo's popularity was fading before the glittering arrival of Raphael in Rome. Young, spectacularly handsome and famously jovial, he ousted the older, legendarily taciturn Florentine from several prime commissions. Michelangelo realised that adopting a younger, better looking, more politically savvy protege than himself would help to compete with Raphael, and thus the odd partnership was born.

Sebastiano is ennobled by his relationship with the great master and, under his coaching, achieves something approaching greatness in paintings like his The Raising of Lazarus on display here. Splash out the extra cash for the audio tour, so you can hear the details of how the mentor's advice changed this, and other, works on display.  Michaelangelo's pencil sketches revealed on the back of Sebastiano's painting of a pieta (displayed facing the cast of the Vatican's sculpture) give you brilliant insight into the raw brainstorming of two artists.

Letters between the two broaden that view, and also show off Sebastiano's diplomatic skills. The cantankerous Michelangelo managed to alienate most people quickly and had few friends. Sebastiano staying in his good graces for almost two decades, even making the elder a godfather to one of his sons, was a remarkable achievement. The letters make clear that the way to Michelangelo's heart was flattery mixed with complete adulation. The two men finally fell out when a more secure Sebastiano started to offer suggestions to the older man's work.

The insight into the creative process goes beyond the dual collaboration. In my opinion, the best thing here is the unfinished Taddei Tondo (which only had to travel down the street from the Royal Academy), because the curators guide you through the whole process of creation. From considering marble choice to comparing the rough chisel marks with the smooth completed areas to observing the innovation of the composition, the work rewards lengthy contemplation steered by the audio guide.

The blockbuster loan, however, is the Risen Christ normally resident with the monks of San Vincenzo in Bassano Romano. Few outside of that order have ever been allowed to meditate upon the larger-than-life, shockingly-human-in-his-nakedness Christ leaning casually against his cross like a Greek god. Michelangelo almost finished this one, but abandoned it when a crack flawed the marble. Another perfect plaster cast stands beside it, allowing us to examine the version he did finish, now standing in Rome's Santa Maria Sopra Minerva.

Amongst the copies, however, I found most delight in the Borgherini Chapel. The quality of the duplication is extraordinary; the ability to experience something this close to a Michelangelo fresco without travelling to Italy unprecedented. While the Sebastiano did the painting, Michelangelo sketched it all out for him first. The contorted figures in the lunettes above are straight out of the Sistine Chapel ceiling, and the Flagellation of Christ is a tour de force of perspective and pathos.

Michelangelo and Sebastiano is on at the National Gallery until 25th June. If you're interested in artistic collaboration, or want to get close to one of the Renaissance's greatest masters without trekking to Italy, get this in your diary.