We shot a scene where we placed a light above a fan, and that created a cool flicker/strobe effect. Unfortunately, there was a couple of shots where we forgot to turn the fan on and therefore they lack that effect.

Any suggestions on how to achieve that flicker/strobe effect in post?

Hi Joe. Is it possible to get a look at a bit of footage where the fan is operating to be able to see exactly the effect you have already? Do you use After effects?

If your using AE,
Maybe placing an adjustment layer above your footage, and adding levels.
Adjust the levels to the brightest flash you want.
On the adjustment layers opacity use the wiggle expression. wiggle(2,50) [example]
If you need to isolate the flashing use a mask on the AL.
You can also hook up sliders to the two wiggle parameters so they can be keyframed.
Jon

Joe... OK I think this should work out for you...

Just get a fan, a light source pointed at a white wall and get the fan into the light
path and adjust the speed of the fan for matching your other footage....

now with your camera shoot the wall and get the strobe effect onto the camera...

now in post you just put the strobed footage on top of the footage you want to have
strobed... now you can set the layer to overlay and adjust the opacity of the strobe
footage until the output matches...