Set Your Frame Rate
It seems so fundamental to editing, you might see no need for discussion of setting your frame rates for imported footage in your edits. And yet, have you done it lately?
Now that the shaming is over, review this refresher/primer regarding your frame rates, unifcation, and how to spot issues that result from ignoring this cornerstone of digtal video.
Set Your Frame Rate Upon Import
A best practice, and terrific habit, is to set your frame rate immediately after importing. Your footage will automatically be selected in the project bin and the following keystrokes will put this out of your mind.
After Effects: Cntrl-Alt-G
Premiere Pro: Right Click - Modify - Interpret Footage
Set Multiple Clip's Frame Rates Quickly
Once you set the first clip's frame rate, you can copy it's properties to other footage without having to re-enter the Interpret Footage properties panel. Select the clip with the desired properties (frame rate) and then:
After Effects: Cntrl-Alt-C Copies properties
Cntrl-Alt-V Pastes properties to all selected clips at once
Of course, you have to use 24fps footage in a 29.97fps comp or sequence if contains speeking actors or instruments being played to the beat of the sound track. Other footage, like vehicle rolling footage, will not suffer from changing it's frame rate. Once the footage rate matched the comp, you will eliminate the stutter that results from every animated element with mismatched frame rates.
In this example, 24fps footage is panned and scanned within a 30 fps comp. Essentially, the position is animated to move the car across the frame.
Now, watch the guard rail along the left of the footage. Why does it stutter. For every second of the 30fps comp, there are only 24 frames of source footage. To fill in the missing frames, After effects duplicates every fourth frame to become (also) the fifth frame. While the fifth frame repeats without change, After Effects moves the footage along its animated path one frame's worth of motion, as it should. So, the layer moves while the layer's content is "frozen." There's your stutter. You don't notice it as badly when the footage is not moving. Watch the above example enought times, and you'l realize, the whole frame is stuttering 5 times a second, and it becomes terribly annoying.
Here's the same footage at 30fps in a 30fps comp. No stutter.
Now you have the ability to fix those camera pans that just don't look smooth. Spoiler alert: you will now spot this error on Netflix constantly where you didn't notice it before.
This problem becomes more pronounced when the footage is paired with a graphic or border. Here's that 24fps vehicle in a 30fps again. The vehicle sits still while the mask and layer position move to reframe it and keeep it on one place.
Also you can spot mis-matched frames rates like these 24fps clouds behind this 30fps pup, when they are animated together. The dog moves smoothly while the clouds stutter.
If you don't want the clouds to move faster by remapping them, then enable frame blending in your switches column, and make sure it's enabled in the global switches above the switches column headers.
Try checking the box once and you get Frame Mix Blending that cross fades the adjacent frames to create the missing frames.
Click the box a second time and you get Pixel Motion Blending. AE actual motion tracks regions of the frame to create a blur map that closely follows the details and blends them between their starting and ending points in adjacent frames.
Pixel motion looks better, but takes longer to render. Here's that dog, even happier over his pixel motion-blurred-floor. No More stutter, but the clouds still get to move at their original pace.
Note. Time remapping 24fps footage so that it is faster by 20% will not remove the stutter. And, neither will Frame Blending it after it is time remapped. Until you change the frame rate in the project bin, there will always be repeated frames in the footage.