how do you....

Switches again, then.

I like Conditional branches, but BC seems to prefer new pages. Personal choice, really.

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Cutscene - Hero has to step on an event to set it off first. So on that event have a second switch turned on.

Then, in front of your door, have an event that is a conditional branch:

If Switch [0001] ON
<>
Else
<> Input Number yada yada yada

Then once the cutscene is over, have the switch turned off (stick it at the end of the event that controls the whole cutscene).