Pathologic Classic HD

You need to see all 11 performances, regardless on which character you see them.

The Bachelor normally CANT see the performance at Day 9, because he is attacked by butchers, that is why, for most, the guide above is the way to unlock the achievement. In theory, you could unlock it on the bachelor route by not going to the abattoir on Day 8 and going to the Theater on the start of Day 9, reloading, then continuing the story and going to the theater the following days, unlocking it at the start of day 12, when you watch the final performance.

You can use Day Saves and watch each individual performance and it should count, its like an internal counter that tracks when you see each nights performance and you need to see all performances for this achievement to trigger.

As the Haruspex, I watched Day 9 and Day 10 performances and it didnt unlock, then I went back to old saves and watched the plays from those but I was missing some days, I just gave up and finished the Haruspex story and started the Changeling's, determined to watch every play on her playthrough until it unlocked, and unlock it did, on Day 8 (after Day 7's events).

So, its confirmed that you need to watch each Day plays, regardless of which story you are running.

Anyway, if you missed Day 9 with the Bachelor, when you see it with the Haruspex, it will unlock.

If you are playing the Haruspex as your first run and need to watch the performance on Day 5 with the Haruspex:

To prevent being caught, make sure the clock turns over to 00:00 inside the Boiler Room after talking to Rubin and completing Day 4 main quest.
Do NOT go outside until its officially Day 5.