Olivier Messiaen and Oliver Latry
Thanks to Bruno Nettl, my post Unpacking “improvisation” covers a lot of ground. Apart from the Usual Suspects (jazz, Indian raga), improvisation was commonly practised by composer-performers in early WAM (revived by Robert Levin in his incarnation as Mozart!); and Nettl goes on to explore the whole creative continuum from preparation to performance, considering Persian “classical” music, Native American songs, and so on.
My post Playing with history features Messiaen‘s Messe de la Pentacôte—the written, reified result of his lifelong practice of improvising at the organ of the Église de la Sainte-Trinité in Paris. There you can hear it played by Oliver Latry on the organ of Notre Dame. 
Indeed, Latry continues the tradition there, as you can hear in many YouTube items. Of course this isn’t just disembodied, “autonomous” music: the videos often remind us that it’s an integral means of serving ritual, at some remove from the secular context of the concert hall. Such a majestic way of “rousing the sacred hall“! Here (under “The reform era”) I also mention the all-encompassing sound of the related sheng mouth-organs in church during the Li family Daoists’ tour of Germany.
While the hectic, dense, monumental fortissimos leave a deep impression, the art requires contrast, with sparse, reflective passages—as in this wonderful improvisation by Oliver Latry in 2011:
Also from 2011, during Sunday Mass—reminding us of the religious context:
and this is VAST:
Here Latry improvises on the Marseillaise for a Mass to commemorate the victims of the terrorist attacks of 13th November 2015:
Another finely-constructed piece for Sunday Mass in 2007:
Messiaen’s musical language as performer-composer remains unique. My posts on Messiaen mainly feature the great orchestral works (Turangalîla, Des canyons aux étoiles, Éclairs sur l’au-delà …, and so on), but his grounding/transcendence in the organ makes an essential background. Amongst his numerous organ works, here’s the early Dieu parmi nous, finale of La nativité du Seigneur (1935)—a perennial favourite:
Let’s end with a precious film of Messiaen himself in 1985, aged 76, improvising gloriously on three scenes from Puer natus est at the Saint-Trinité organ—after over half a century there:
Messiaen introduces his meditations in turn:
- Les bergers dans les champs voient apparaître une troupe d’Anges qui chantent Gloria in excelsis Deo
- Et les Mages avaient vu l’Etoile du Christ en Orient et se sont mis en route vers Nazareth [or rather, Bethlehem!]
- Et les Mages offrirent un présent à Marie et à l’Enfant Jésus de l’or comme un Roi, de l’Encens comme un Dieu, et de la myrrhe comme un homme mortel.
 The Notre Dame organ was spared from the 2019 fire. Click here for an engaging survey of Notre Dame’s whole cultural significance. For the Li family Daoists before the cathedral, see these engaging notes on our 2017 French tour.