Since Defender updates are delivered with Windows Updates, then just like other Windows Updates, if you have it set to Notify Only, it will wait until you OK it.
From the Windows 8 Help and Support regarding Defender:
To help keep your definitions up to date, Windows Defender works with
Windows Update to automatically install new definitions as they're
released. You can also have Windows Defender check online for updated
definitions before scanning.
According to that, you can also have Defender do an (extra) check for definitions before starting a scan but. There is no UI control for that available, but you can easily modify how the scheduled scan is launched and with what options Windows Defender is working. The system calls
MpCmdRun.exe, which has quite some command line flags. Here is how you modify it to update the signatures before the scheduled scan:
Scheduled Tasks UI and search in the library for
Change the command line flags (for the task
MP Scheduled Scan) to:
SignatureUpdate Scan -ScheduleJob -WinTask -RestrictPrivilegesScan
Done. Now Windows Defender will update it's signatures before it does a scan.