The displays wont do a thing till the MPU is sorted.
The flash sequence will give you an indication of what MAY be wrong. The flash codes are detailed here.... take the diagnostics with a slight pinch of salt. For example, if you get a 3rd flash but not fourth then the PIA itself may not be defective but its logic voltage may be or its ground etc.... flash sequence basically points you in the right direction.
As Nino said, it will flicker first and then go through the flashes.
First job of any non working game is to make sure the power supply is running properly... get this going and move on from there

First Flash:
 Signals the end of the ROM test.


 Second Flash:
 Test the NMOS RAM (U7). There will be a bit of a delay between the first and second flash, be patient. If no flash, replace U7.


 Third Flash:
 Tests the CMOS RAM (U8). If no flash, replace U8 - use caution the 5101 chip is NOT diode protected and is very static sensitive!


 Fourth Flash:
 Tests the first PIA chip (U10). If no flash, swap U11 for U10. Note that although you may get the fourth flash, U10 may still be defective.


 Fifth Flash:
 Tests the second PIA chip (U11). As above. If you have already swapped U10 and U11 and don't have the fifth flash, try a new 6821 in U11. If still no success, swap once more. Worst case-replace both PIA's.


 Sixth Flash:
 Tests the Display Interrupt Generator. This actually tests both U11 and U12. If U11 has been replaced, and no sixth flash, replace U12.


 Seventh Flash:
 Tests the "Zero Crossing Detector". This tests U10 and U14 as well as the presense of +43 volts from the rectifier board. If no seventh flash, check for +43 volts before changing anything else. Fuse F4 (+43 volt) is a common failure. Unless U10 has already been swapped for a new or known good 6821, change it before U14.