A musician can practise their own individual part until it's flawless in isolation and still struggle to actually perform well as part of an ensemble, because playing well individually and playing well as part of a group are genuinely distinct skills. Ensemble playing requires continuously, actively listening to everyone else in the group in real time and adjusting one's own playing, tempo, dynamics, timing, in direct response, a skill that practising one's own part alone, however thoroughly, never actually trains or tests.
A part practised in isolation gives no experience of adjusting to others
When a musician practises their individual part alone, they naturally settle into their own internally consistent sense of tempo, dynamics and phrasing, entirely undisturbed by anyone else's playing, since there's no one else present to disturb it. This isolated practice, however extensive, provides no actual experience of the specific skill ensemble playing demands: continuously monitoring what everyone else in the group is doing and making small, constant real-time adjustments in response, something that simply doesn't arise as a challenge when practising completely alone.
Ensemble listening means tracking the whole group while still executing your own part
Playing well in an ensemble requires a musician to divide their attention between two genuinely demanding tasks simultaneously: executing their own individual part accurately, and continuously listening to the group as a whole to catch and respond to small deviations in tempo, dynamics or timing before they compound into a larger ensemble problem. This divided, continuously adjusting attention is a distinct trained skill in its own right, which is precisely why experienced ensemble musicians and conductors treat rehearsal time spent actually playing together as genuinely irreplaceable, rather than assuming that individually well-prepared parts alone are sufficient to guarantee a well-coordinated group performance.
What we're still unsure about
That individual proficiency and ensemble proficiency are genuinely distinct skills, and that ensemble playing specifically requires continuous real-time listening and adjustment, are well established, widely recognised findings in music pedagogy and performance research. What's more genuinely an ongoing area of instructional debate is exactly which specific rehearsal techniques and exercises most efficiently build this ensemble-listening skill in developing musicians, since different music educators and ensemble directors report real variation in which approaches, from smaller sectional rehearsals to full-group run-throughs, most effectively build this specific capacity, without one universally agreed optimal training method.
This sits inside Ensemble Playing & Listening, one of seven topics in Performance, one of four domains in Music, one of seventeen subjects the app can quiz you on.