I think in those days of the Vic-Wells, especially after the war, it was a case of "all hands to the pumps", Mr Y. But a sensible singer can always turn in a good performance, if they sing it with their own voice and don't try to mimic Alfredo Kraus
For example, a forgotten moment in the annals of ENO was the tail-end run of BOHEME which had Graham Clark as Rodolfo, and Josephine Barstow as Mimi
Clark hated doing it and was in a bad mood for a month, but it was certainly the most animated Rodolfo you'd have seen there at the time
Against that, in WAR & PEACE, Pierre had always been done by Kenneth Woollam, and he'd made the thing so much "his own" that he was entirely identified with it. And then, in the last week of the season, Alberto Remedios went sick in TRISTAN, and Woollam was moved in to cover - no-one else could do it. Obviously he couldn't sing Tristan-Pierre-Tristan-Pierre-Tristan on alternate nights... so Stuart Kale... who had covered Pierre for about five years and never, ever, had a single show of the thing... finally got to do Pierre. How would a Mozartian tenor with a talent for operetta fair in the colossal heroic role of Pierre? Stupendously, it turned out. A lot of it comes down to artistry in the end.
