I remember in the 80's when this Ad was everywhere, on billboards and bus benches and magazines and newspapers. Pretty sure it won some awards.
The TV commercial version was the same woman smiling, just a slow zoom in towards her smiling face, but there was this quiet static in the background with eerie footsteps coming closer for about 15 seconds before the audio dropped to total silence for a few seconds, then a narrator (James Earl Jones-style) said the motto, and the woman said the bit about the free fries, followed by another 5 seconds of silence while she kept smiling; but the smile was cracking, fading, her face muscles unable or unwilling to hold the pose until they suddenly dropped into a look of terror right as the commercial ended.
I'm willing to be proven wrong but this ad is almost certainly fake. For one the logo used in the ad didn't exist? In the early 60s McDonald's did have red text but it wasn't off-center, when it was later off-center the text was white or black but not red. Maybe that's the joke and why it was created, but McDonald's was never this edgy. It was more commercials about mcnuggets being born to die or the weirdness of the hamburgler and grimace.
Her eyes and mouth are slightly wider than a relaxed expression, so there's visible tension. In video it could be cute, like she might just be happy, but if you freeze just that one moment then her expression is ambiguous. Either she's talking and smiling enthusiastically, or she's about to eat you enthusiastically, or more realistically she's afraid and trying to hide it. Add the creepy text and you're primed to interpret the expression negatively.