Well, if you look at the schedule of the old repertory companies (common in cities and big towns before TV and cinema essentially killed them off in the 60s -- theatres with a resident company that performed a bunch of different plays in rotation), they were pretty demanding.
Some companies presented a different play every night, with everyone taking one or several parts and understudying several more.
Weekly rep meant that there was a new play every week, so that week's play opened on Monday night and ran nightly, with an additional matinée performance on Wednesday and Saturday, while, during the daytime, they learned their lines and rehearsed for the next week's play.
On Sundays, the only day there were no performances, the crew dismantled the previous week's set and built the next week's set, hung the lighting, set up sound etc. On Mondays there was a tech rehearsal in the morning and a full, costumed dress rehearsal in the afternoon with lots of notes from the director, and the new play's first performance was that night.
On Tuesday, the cycle started all over, with them giving the second performance of that week's play at night and starting rehearsals for the next week's one in the day.
And although they only started rehearsals for a new play on Tuesday, actors were expected to know their parts and be off-script by the Thursday.
Plus being expected to understudy several parts other than your own.
So you could be playing Rosalind in As You Like It, plus understudying any of the other female parts, while simultaneously learning your lines and rehearsing Cecily in The Importance of Being Earnest while understudying Miss Prism and playing any number of servants and walk on parts in both plays.
You really needed to want to act.